I had a bunch of other things to deal with myself, but now I can offer some further results and I'm quite irritated still and wonder what the heck I might do that is different since it does not seem to effect others:
- I had a completely different machine (different hardware) installed two weeks ago, exactly the same issue. In this case it was a bit of a problem, since it went to a friends place, so being able to log in remotely was kind of important. In the end I couldn't really fix it before we had to take the machine to his place. Here is my log of what I did:
1. install with freshly downloaded openmediavault_6.0.24-amd64
2. install bunch of cli-tools (I'll go into that later, not the issue even though I suspected those first)
3. installed plugins (backup, borg, diskstats, mergerfs, resetperms, rsnapshot, sftp, snapraid, symlinks, cputemp)
4. formated HDDs, created shared folders and activated SMART checks
5. backupjobs for rsync and rsnapshot
6. at that point I noticed the notifications aren't sent, I had that issue before, that after a while they suddenly appear. I guess this is another issue.
7. dashboard activated, set userpassword for webinterface, added two users.
After a restart I again could
- login to the webinterface with no issue
- login locally on the machine with root
- could NOT log in with SSH
- after changing the root-password locally it works again, until the next restart.
I originally thought it is related to the complexity of the password, with easier passwords it seemed the problem does not occur, but that was false. See below.
So the strongest indicator what somehow creates the problem is the restart. (??!??)
Now today I had time again to fiddle with the three machines I had the problem first with. And this is weird, stay with me:
I intentionally did only very rudimentary things and logged everything yet still I get the same problem:
1. install with freshly downloaded openmediavault_6.0.24-amd64 (on three identical machines, installed with three separate USB-thumbdrives, but created from the same ISO)
2. NO cli tools installed at all this time to be sure it's not them.
3. loaded updates via webinterface
4. plugins: backup, borg, rsnapshot - nothing else
5. settings: set timezone, logout to an hour, powerbutton to shutdown
6. restart
--> same fucking shit...
During the installation of updates I got this error which I'm pretty sure is not related:
Setting up Salt environment ...
[ERROR ] Command '/usr/bin/patch' failed with return code: 1
[ERROR ] stdout: patching file /tmp/__salt.tmp.lv7pru4c (read from /lib/python3/dist-packages/salt/fileserver/roots.py)
Reversed (or previously applied) patch detected! Skipping patch.
1 out of 1 hunk ignored -- saving rejects to file /tmp/__salt.tmp.cr2of4lk
[ERROR ] retcode: 1
Solution / sort of work around:
because it seemed just unreal I tried again a couple of different ways to log in and varying ways to insert the password:
- remember, locally always no problem, only via SSH the login fails
- I safe the passwords in a passwordsafe / during installation in keepnote
a) if copy and paste from keepnote --> does not work after restart, but once changed the password locally it works again (????)
b) if instead of GUI for copy paste I use strg+shift+v -> same result, no login
c) if I type it in by hand --> no problem (which sucks, since I use rather complex passwords) but at least I can log in now
d) Now the funny bit: I type the password by hand in another line (directly below to make sure there's no typo), copy it --> NO login
e) AFTER I once had logged in by typing by hand THEN it works again with copy and paste, no matter of GUI or strg+shift+v
It took me a while to get through all the possible variants. And it fuckin sucked!!! 
But I guess this is not a OMV issue. I'm using Debian Buster on the client, might be that one (unlikely, since it only occurs with OMV6 machines) or maybe some weird thing how debian 11 deals with copypasted passwords.
Does anyone of you have regular contact to someone in the debian-universe to investigate this further?
If nobody else has had this issue it may mean
a) everybody instantly switches over to keys once installed (unlikely)
b) everyone uses way to simple passwords and don't mind typing them (I hope this is unlikely but am not sure)
c) there's something else involved I haven't figured out yet (might very well be)
In any case, this was not fun. I'm gonna get some chocolate ice now. And rethink my choices... 