Dear @Sharon Zhao-MSFT ,
thanks for the response.
I just checked whether the problem is still there, but it is not. I have changed absolutely nothing. Like a miracle. I don't quite get it, but I can log in now.
There seem to be many people with a similar problem online:
search
With varying ideas for solutions. I tried a few but it did not work, which is why I had come here. I hope the problem does not reappear again.
edit: I seem to have written the original post with my other account. I am the opener of the thread.