Unfortunately, a hard reset did not solve the problem. For testing purposes, I switched back to the normal Fairphone OS (with Google Apps) where user switching works as expected. I then reinstalled FP Open OS, but the problem still exists.
As far as I can tell, the Fairphone Launcher is not started when I switch to a secondary user on FP Open OS. After switching, I get the message that the Setup Wizard does not respond and whether I want to close it. If I press "ok", the full navigation bar appears sometimes, and I can open the settings via the message bar. There, I can see that the launcher did not start (it is not visible neither as an active app nor in the cache).
Does anyone have a suggestion what to do?
P.s.: In the settings of the secondary user, there is the option "Home" with the possibility to switch between three launchers, "Fairphone", "Fairphone Launcher 3", and "com.android.provision.DefaultActivity".
- P.s.: I only realized it now, but I can of course also access the settings via the lock screen.