Devicenet error codes
Error 78
I recall that the most likely cause of a "78" error on DN is that a DN node is simply powered-off somewhere. On the scanner module, the offending node number should follow the display of "78" in an alternating fashion. If several devices are turned off, then a succession of "78"'s followed by each of these node numbers will display.
Probably, you have a Motor Control Center containing Variable Frequency Drives, each of which is connected as a DeviceNet Node. A Drive is either tripped or powered off, I'll bet, which would as a consequence power-off the associated DN slave module.
Perhaps somebody has locked-out a DN remote rack somewhere, or locked-out a Drive.
A less likely cause is that a DN slave module has died, which I have seen a few times as well.