Pre-upgrade checks?

I first installed UCS with version 2, already, and upgraded ever since.
A new install was only ever necessary during my first tries.

I replaced a Windows DC with this and used Kopano to replace an Exchange server and installed also Nextcloud to replace a no longer supported Windows version of Owncloud.

So, all in all, this is a very basic home use installation, running on Windows server in a Hyper-V environment with only one UCS server, about a dozen computers and VMs (Windows and Linux) and only a handful of family users.

I have been able to upgrade and replace defunct apps so far using the internet and the Univention forums but this time it is far more complicated than in the past.

Next steps I will try is to

The last try ended with the following errors:

I tried to run the command but ran into more errors as per below:

I am grateful for any other hints to resolve the above problems.

I have found a link regarding the PG errors but am hesitant to follow it at the moment:

Thank you very much and a happy weekend to everyone.