So I would like to just tell you what I had to do to fix this and maybe provide some references for people facing similar problems.
Now after I was pointed towards the Debian version being not up to date, I first ran the upgrade by the usual way of changing apt sources and running apt update and apt full-upgrade. This worked, but afterwords I had major issues trying to get apt sources up and running again. The issue was, that the keys of the repositorys were not in the apt keyring. As the keyservers used by pretty much any online guide are not running any more. So I used the Ubuntu keyservers, they worke fine. Btw. The add keys command mentioned in the docs is not working either anymore, seems like the server does not support the command or it is disabled, so it would be necessary to update this page.
However, this lead to a running bullseye Debian on which I was able to install java17.
However, openhab would still not run, I could not get into the console or the Webinterface. After a bit of fiddling around, I stumbled across a thread in here with a similar issue and a openhab-cli clean-cache did the trick.
Now openhab worked fine, but mosquito did not work yet. It got upgraded at some point in my wild adventure of apt upgrade variations to 2.xx. Now the issue was the pid file, whatever this is, I needed to comment out the line in the configuration mentioning it aaaand everything worked.
Now as to weigh in on the update discussion. I am a pretty experienced user of Debian and apt. Nevertheless, the update mechanism of openhab remains very unclear to me. For example, what does openhabian-config do different from apt update and apt upgrade? Also the commands run by this tool are not output to the console and neither is the output. This would make debugging much easier as you could just get more info than failed. Now I am sure there is some file with the output, but as you have to go into the shell anyway, there seems to be little advantage to not displaying the output for quick reference.
Now as a long term goal, I think it should be a simple thing of some Webinterface existing to manage the server and upgrading with every necessary part of it, from the os to various openhab components. This would widely broaden the userbase.
Also I try to keep up to date with the news on a update, but sources are just very unclear. Where is the proper documentation? Is it on GitHub? Or in the docs? Now I am sure this is not a problem for the people maintaining openhab, but it is for the people who are not as familiar with the way open source projects are structured. I would love to contribute to these issues myself, but I just can’t seem to understand how things are supposed to be done.
Nevertheless, I have the deepest respect of all the people keeping openhab up and running and providing such a mature software with such great functionality. Especially considering the way openhab has gone from 2.5., I love using it and working around getting stuff running with the quirky ways manufacturers build their devices.