hello
It seems this is due to your directory connections and some "old" files no more available on server.
- Check if login/password defined in directory connection are correct "General Tab ..use "Test connection" button)
- Try to edit user group and check a "refresh group directory on save"
- If nothing works so as a workaround, You may try to create a new directory connection and then use it in your user groups.
Regards
PS : you may already have performed some of the action above.