All of a sudden a 7940 is getting the message "Protocol Application Invalid" on the LCD. It's impossible to use the keypad, but it is possible to reset the phone to factory defaults by the normal procedure. However that doesn't correct the problem.
I've also tried performing a firmware upgrade. The phone contacts the TFTP server and the file is served. The phone says it's inspecting the load file, then goes back to the "Protocol Application Invalid" message on the LCD.
I found lots of other people having the same problems, without any obvious solutions (as a factory reset doesn't help). Also, I found an article by Cisco regarding the CM software and the dhcp option 150 bug, but this isn't relevant here.
I'm at a stop. What to do? Thanks.