The Forced ALTBOOT no clone no data backup would be pretty much the only option here. This is an issue that applies in your location and as far as I can see, not in others. This most likely was caused by a clone file being used across different devices/firmwares. These are my guesses, but since it happens on your machines but is not being reported in other locations with the same model printers I'm at least pretty confident in it.
My suggestion is to work on one machine. Have a tech paged to do the Forced altboot, no data backup, no clone.
Set up the one printer manually and just import the addressbook through the webui from one of the other 78xx.
Clone it and save that clone.
Then test that one device as long as needed to verify it is resolved. Then have the Forced altboot no data backup done to the rest of the devices. Use the clone from the known good test machine to bring back the newly loaded devices which are all now on the exact same firmware levels with no chance of corruption.
A tech can do the upgrade, you would just need to setup the one device manually, and import the addressbook. In the 2nd wave, the clone from the first machine can be used on all the other 78xx printers that are altbooted. The reason we typically state no clone is to eliminate the idea that the "glitch/bug" is getting cloned, which can/will/does happen. This clone would be a known good one from identical firmware on identical hardware.