Thanks. Concerning the links: Ok, if you say so. At the beginning I have testet some things ans items, which I deleted or changend later. So it is possible that this are remains of it. Strange enough that these items only caused problems after the release update. In the old release (2.3.0) it did not happen.
Anyway, it seems like a bit of work for me, as I have also the Error Code 500 and 503 Problem, that is documented here.
https://community.openhab.org/t/error-500-and-503-after-ver2-4-upgrade/62845
So I will try to uninstall and reinstall all bindings, one after the other and check if I get rid of it. Maybe also the add ons like the myopenHAB cloud.