ssh regenerate key works for me.
tested on 50+ problematic boxes.

could not connect to old routeros (4.x, 5x) - winbox 3.12 works fine
ERROR: coud not fetch index

i did backup with error "backup,critical error creating backup file" and restore box with this "error" backup. config is without change and error dissapears... tested only on few devices, some ok, some not btw i tested in lab upgrade from old ros (4.10(netinstall)->5.26->6.40.8->...

same downgrade from 6.42.3 to 6.40.8 (bugfix) (because has error with backup: "backup,critical error creating backup file: could not read all configuration files") RouterBOOT booter 6.42.3 RouterBoard 1000 CPU frequency: 1333 MHz Memory size: 512 MiB NAND size: 64 MiB Press any key within ...

i updated about 200 devices (mipsbe, powerpc, tile, x86, mmips) to 6.42.1 and about 30% has problem with backup: 10:44:06 echo: backup,critical error creating backup file: could not read all configuration files i test one device with this error (RB1000) and enable all packages, update firmware, upgr...