My "entry" account is SO. I can easily switch to MSE, Programmers, Mathematics. But I can (edit:) no longer switch to AU. That is, the Ask Ubuntu page shows, but I am not logged in (and the system suspects me being a robot). For reproduction, I go to my profile, and select the link to my hp.
For MSE, Programmers, Mathematics I am logged in as expected (as is shown in the black-bar).
What did work was to login manually into AU as suggested by @AnnaLear. Since then, I am logged in. Will probably take some time to see if this is a permanent solution.