Problem: Domainjoin of school slave fails

Problem

A join of a new school slave fails. In the join.log you find this line:

... /etc/ldap/slapd.conf: line 136: unknown attr "@univentionVirtualMachine" in to clause

Solution

Do not install any packages during installation and do not join a new school slave not during the installation. Restart the machine an do the join manually.
Check if univention-virtual-machine-manager-schema and python-univention-directory-manager-uvmm are installed on the (fresh) school slave

dpkg -l |grep univention-virtual-machine-manager-schema
dpkg -l |grep python-univention-directory-manager-uvmm

If these packages are installed and you receive this error, purge this packages by

apt purge python-univention-directory-manager-uvmm univention-virtual-machine-manager-schema

Now you can join the slave by executing:

univention-join

Root Cause

It is a actual bug