Windows account sign-in damaged; shows empty window and refuses the sign in

Weird problem here. Win11, latest updates installed, build 26200.7922

After RAM related system crashes because of a damaged module (replaced by now), I am now having problems to log into the store using my windows account.

I first encountered it when the windows store told me an "app wasn´t available" and when I opened the store, I was locked out of my account there. I click sign in, account is there, I click on it, and then "continue", but then it´s just an empty window and no sign in occured - BUT purchases are there. When I repeat that several times, I can even access the apps again. Weird!

Next thing is - I changed my account to a "local account" and can´t bring it back being an online / MS account. Sign in button (under settings < accounts ) is there, but when pressed, it displays an empty window and nothing happens.

I also can´t add another user via settings > other users . Again just an empty window.

DSIM and sfc report no errors.

reset to an old version a week ago doesn´t have any effect.

In-place repair doesn´t help.

Any ideas?
Thanks in advance.