I feel your pain (Unable to sign in to XBox (Steam version on a PC ) - #3 by trgz)
TLDR version: if you get that unable to sign-in again, then it seems that Alt+Enter (simply changes it to windowed?) might allow you to pick an account/sign in - it worked for me the second time it happened.
I’ve also learnt not to validate my MSFS game files in Steam (you lose the lot) 