I'm having this same error! That article you referenced offers NO HELP - it hardly explains the problem and then recommends starting a new return.
FWIW, I'm using a PC that has been reset/reinstalled since the last opening of both the primary and backup 2021 files. Have no idea if I at some point inadvertently tried to open another file type, but fail to see how that could corrupt both primary and backup files, especially when (if one's gonna try to open the wrong file type) neither would've been open in the first place.
I've also tried installing TT on another PC and same error. In both cases, TT accepts my password before the error.
There's got to be a REAL solution to your problem.
