Where are the plugin files? Where to write a plugin if I get it like 7z archive?

All plug-ins for AutoCAD are in the folder
Each plug-in there own folder with the extension .bundle
AppData folder is in the user's profile and is marked as hidden. If you do not see the AppData folder, it means that the Explorer does not show hidden files. You can simply type appdata in the address bar or go to Explorer settings and enable the display of hidden files and folders. Or just copy path 
and past it in Windows Explorer address line.
You can install a new plug-in for AutoCAD simply writing files in ApplicationPlugins. And for remove any plug-in, you can simply erasing the “.bundle” folder (when closed AutoCAD course). AutoCAD constantly monitors what is in the folder and itself loads the new plug-ins. At the same time it copies menus-files to folder Support. It is also sometimes necessary to clean from unnecessary menu-files (.cuix and .mnr)

I installed the plug-in and see new buttons in AutoCAD. But click on any of them leads to the error message "Unknown command xxx".

This problem is artificially invented for you Windows developers. Any file downloaded from the Internet can be marked as blocked. A blocked library AutoCAD will not load. Therefore, you will see a button, but you cannot run the command from the blocked plugin. So that Windows could not lock the files I publish 7z-archives, rather than ZIP. But this does not always save. You will have to find a locked file, go to properties and click the Unlock button. The main library is in each plugin directory
Locate the file, open its properties, and check whether the Unlock button does not appear on the first tab of properties. Click it and restart AutoCAD.

I upgraded the Autocad to the new version and all my plugins disappeared somewhere.

Yes, when changing versions of the AutoCAD and even just when installing the service pack, the internal number of AutoCAD changes. And all the plugins in the PackageContents.xml file are written, with which specific versions of AutoCAD versions this plug-in works. You will have to urgently change all the plug-ins along with the Autocad. I try to update my plug-ins quickly and in a few days (if it does not happen terrible), then you will see new archives with new versions on this site. With the App Store, things are much worse. There the publication of the new version takes a month or more.

But in most cases you can patch the plugin yourself. After Autodesk has not changed anything in AutoCAD for a long time, it is not necessary to reprogram plug-ins, it is only necessary to change the version number. Try to do so: find the PackageContents.xml  file in the plugin folder, open it for editing (this is plain text, you can edit it in Notepad) Start the search for the SeriesMax parameter and replace it with a new version of the AutoCAD everywhere. When upgrading Autocad 2019 to AutoCAD 2020, find SeriesMax="R23" or SeriesMax="R23.0", replace with SeriesMax="R23.1"

If such a substitution of the numbers does not work, then you will have nothing to do but wait for the release of a new version of the plug-ins.

Every time AutoCAD starts, it displays the message "AVС_ххх.dll File not found". But everything works.

This message occurs if AutoCAD searches for a long-deleted plug-in. It turns out AutoCAD itself writes some plug-ins into autoload via the Windows registry. It's easy to remove. It is only necessary to find unnecessary records. Open the registry editor on section
(Revision R22 and localization number 409 of the AutoCAD can be different in your system)
And find there all the subsections in which there is a LOADER key containing the path to the AVC_ххх.dll file. 

A subsections can have any name. But it must be removed entirely.
You can safely delete all the subsections from this registry sections, because all A>V>C> plugins will load from the bundle folder.

Autocad 2019 instead of pictures draws a cloud on buttons of classical panels.

This is a known problem with this version of AutoCAD. You must install AutoCAD update 2019.1 or newer.

Plug-in not work in AutoCAD 2017 SP1. No buttons shown, no command work.

Try Autoloader patch

I have only a dozen solids in model but LAY and SAW command freeze on a minute!

Try to call for the same object command MassProp. My programs use this command to calculate the volume of a solid. This command must return a result immediately. If this command will work too slowly, it means that you use solids with extremely complex shapes. To complicate the AutoCAD works unbearable freeze just any solid having splines on the edges. Such complex solid appear, for example, when you trace the picture with a spline and then extrude a spline to solid. Or when you import objects from other programs. Even if you are not using my plugin, you still will not be able to work with such objects. I highly recommend to explode it and cut around approximately the lines and arcs, in any case not using splines. Please note that when you merge lines and arcs into a polyline with the JOIN command sometimes makes a spline instead normal polyline. Will have to Tinker - do not use JOIN, draw a polyline and extrude again. You will get a performance gain in tens and hundreds of times, will save your time and nerve cells.

Why do I see #### instead of the name and size of the part?

If you see the #### characters inside the texts or tables of AutoCAD, then you have used the FIELD and AutoCAD for some reason could not calculate the field values. If the field is not automatically calculated, then the user may have corrupted the FIELDEVAL variable settings. To update the fields manually, use the _UPDATEFIELD command. To disable the fields in all AVC command you can use the main Common Options of AVC. The SAW command has its own "Use fields" checkbox for each Sawing style.

I installed the new version of the plugin, but the new buttons are displayed, all of the buttons from the old version

Close AutoCAD. Remove AVC_xxx.bundle plugin folder.
Next, go to the folder "Support" 
c:\Users\<login>\AppData\Roaming\Autodesk\AutoCAD 2018\R22.0\enu\Support\
In this folder, it is necessary to find files and AVC_xxx.CUIX AVC_xxx.MNR and delete them.
Start AutoCAD 1 times. You should see that the old buttons have disappeared. 
Now close AutoCAD and install the new version again.
If you created your own ribbon or toolbars or workspace or drag the plug-in command on the Quick Launch toolbar, you may have to adjust all manually again for each version of the plugin. For this reason, I try not to change the names of commands and keys figures. And if I do this I'll warn users in the description of the new version.

I run the plugin installer, downloaded from the Autodesk App Store. I checked that there was a plug-in folder in the folder ApplicationPlugins. But it does not work; new buttons will not appear. 

Alternatively, it is possible that you have is not a pure AutoCAD, one of the add-ons (verticals), Architectural, for example. I test the plugins in pure AutoCAD and AutoCad Mechanical. And I cannot guarantee that my plugins will work correctly in other AutoCAD verticals. So I mark plugins published on Autodesk App Store only for these two options. And you will not see my buttons and commands in Architectural, Electrical etc. If you still want to take the risk, you can download the 7z archive from my site. In the archives I have always observed all verticals. Anyone who tried to use my plugins in Architectural satisfied, everything worked.
You can also self-prepare any plug-in for any verticals. To do this, you need to find PackageContents.xml file in the plugin folder, open it for editing (it is plain text, you can edit in Notepad)
Start the search parameter Platform = and replace it with the value of AutoCAD|ACADM to AutoCAD* . The asterisk means that the plug is suitable for all verticals. This text should appear in a file many times. It is necessary to replace all.
Save the file and start your AutoCAD vertical.

Some commands does not work if SolidHistory on. 

History of Solid - this is a big problem. Many commands do not work with these Solid. Sometimes you cannot select the right solid-face. I strongly recommend that you always set the properties of solids "History" = None. The best way to fix it - written in your DWT-template system variable SOLIDHIST = 0. This also will save you from a huge DWG-file and terrible AutoCAD brakes.

Sometimes I see some empty buttons in A>V>C> toolbars and ribbon panel. Or instead of the icons a cloud is depicted.

  • Close AutoCAD
  • Go to folder 
c:\Users\xxx\AppData\Roaming\Autodesk\AutoCAD 2018\R22.0\enu\Support\
  • Delete all AVC_xxx.cuix and AVC_xxx.mnr
  • AutoCAD will create them again at startup.

When I use Smart Leader I cannot seem to force it to read the part number that’s assigned by the SAW/LAY tool. I’ve tried the %n% script and neither works. Is there a way for it to list the part #?

The numbers of items that the LAY and SAW command can output  (%#% substitution) are not saved anywhere. You will not be able to use them in Smart Leader. But it's not a problem. Use the Names and Numbering plugins to give the name=number for each part. I specially created these programs to solve your problem (It all included into the package AVC_KIT) Now you can display this number in Smart Leader using %n% substitution. And you can use properties Kind and Info for long description of part. They accessible in Smart Leader too (use %k% and %i% substitutions).  

How to replace the plugins AVC_Kit and CNC_Prepare with AVC_Pro?

You will have to undergo the procedure of re-activation. No donations are required (but not forbidden, of course)
  • Close AutoCAD and delete all AVC_xxxx.bundle folders from the ApplicationPlugins folder
  • Download from my site file AVC_Pro_xxxx_xx.7z
  • Unpack the contents in ApplicationPlugins
  • Start AutoCAD and in it any command from the menu A>V>C>
  • The activation dialog is displayed. Click the Send Request button
  • The form for sending the request will open. Your e-mail should already be written in the form. Enter a note: "Replace AVC_Kit and CNC_Prepare" and click Send Request
  • Wait for the new key to be received on your mail. Do not forget to check the mail and, possibly, the Spam folder
  • Enter the key in the activation window when it appears.