Problem:
Your systemdiagnostic shows the following traceback:
Investigation:
Our groupfilter in /usr/lib/python3/dist-packages/univention/management/console/modules/diagnostic/plugins/908_ucsschool_school_admin_accounts.py uses
GROUP_FILTER = "(&(univentionObjectType=groups/group)(cn=admins-*))" and finds all groups named as admins* . Groups like cn=admins-nextcloud are also found, but does not have anything to do with schooladmin consisty
So these ldapsearches may also help to find users and groups missing school attributes
univention-ldapsearch -LLL '(&(objectClass=ucsschoolType)(!(univentionObjectType=groups/group))(!(ucsschoolSchool=*)))' 1.1
univention-ldapsearch -LLL '(&(objectClass=ucsschoolType)(!(univentionObjectType=users/user))(!(ucsschoolSchool=*)))' 1.1
Solution:
Rename the found group or add the ucsschoolSchool attribute to the found objects. You can do that via ldapmodify