I have made some progress.
I inserted an SD card from a printer that doesn't work into one that does, and the same symptoms appeared. It appears that the printers are not working because of errors on the SD card. The only question that remains is what is causing them, because I find it hard to believe that so many SD cards broke at the same time.

Yes we're aware of this initiative, and we're considering supporting this protocol in the future, probably on Duet 3.