I have just updated a newer version of openhab from 3.3.x to 3.4.1
Everything went like should and I don’t recall of getting any error messages etc.
After reboot everything looks normal, but nothing works. When I open things (…:8080/settings/things/)
I can see, that every single thing is in the state “ERROR:HANDLER”. I have no idea how to recover from this or where to start looking a solution.

Folks are going to need more information How to ask a good question / Help Us Help You - Tutorials & Examples - openHAB Community. For instance what binding(s)?

All of them. Unfortunately I am unable to list them, the page holding the bindings never shows up, It just keeps loading.

Something is broken, but I have no idea how to start troubleshooting this.

I would try restarting OH, not rebooting. Could also go into console bundle:list

What is in openhab.log?