Hello,
Many thanks @ahrnke – after reading your helpful post and performing the upgrade, I also found this KB article 
There were still a few pitfalls:
- Even after pruning all unnecessary kernels, /boot still did not have enough free space. Solved by: Problem: Checking disk_space FAIL
- When upgrading from intermediate stage 5.1 to 5.2, initramfs failed because there was still a deprecated open-vm-tools module (I think it was ‘vmxnet’). I had to deactivate this and uninstall the outdated open-vm-tools.
- In the end, Apache failed to start because the php 7.3 module was still active, even though the ‘.so’ file is no longer present. This was fixed with a2dismod. As far as I understand, these versions of PHP are no longer supported anyway. (Problem:UCS 5.2 - all php packages are removed since the upgrade). Maybe one should remove the remains?
- The unsuccessful attempts 1-60 of ‘Try to download idp metadata’ repeatedly delayed the upgrade process for a some time. Since SSO was never used, I assume that this part is simply not there.
At the moment, everything seems to be running smoothly, except that a ‘kopano-cfg’ keeps popping up now an then in the log files, even though Kopano has been uninstalled for a year.
Best regards,
TP