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 and do the join manually.
Check if univention-virtual-machine-manager-schema and python-univention-directory-manager-uvmm are installed on the (fresh) school slave (also may happen on a backup server)
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 these 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