tart using Ctrl + Alt + S every few minutes.
All very well, but the whole point of the automatic save and backup system is that it should be doing this for you.
OP how frequently is your autosave set to save? If you get this situation have you tried the next “previous bak” file?
Does that file work or do all of them give the same message?
Is the bak that won’t load smaller in size than the cpr or an earlier working bak??