For the second problem regarding the joinscript we will prepare an update for next week in: Bug 55681 – univention-fetchmail-schema joinscript fails when univention-fetchmail package is not installed
The first poblem: you are already updated to USC 5.0-3? Because that check should prevent the upgrade.
Please never use apt install in UCS but always univention-install - you can break your system otherwise.
What is the output of univention-check-templates? and what is the output of dpkg -l univention-mail-postfix?