Bug: cannot login twice in different browsers

Only one login is possible at a time.
If a second browser logs in, then the first session is invalidated.

This seems unnecessarily restrictive, but if it is a design criterion, it should be documented as such.

Start by reproducing the login behavior in two different browsers and trace the authentication or session-handling entry points. The issue does not name files or tests, and its resolution is undecided: allow both sessions or document that the second login invalidates the first.

Written by the indexing model from the issue text.