I rarely do a server upgrade. Having said this I today upgraded Debian 10 to Debian 11 using these instructions which appeared pretty straightforward.
The issue was that after the upgrade some of the previously installed PHP packages were missing. Moreover, MariaDB was no longer there. I mitigated this by installing the missing PHP packages and also reinstalling the MariaDB database server again.
However, I have the feeling that something went wrong with the process which otherwise went smoothly. Any hints on how do avoid such issues in the future?