After trying different plugins for the user management, two of my users active during this experimentation phase always loose their role to "nothing" after login, no matter what role I set. All plugins possibly relating to this problem have been removed and everything works fine for all other users, which have been created after the removal of the plugins. Even deleting the two affected users and recreating them does not help.
Currently, I'm using the "ldap-authenticator" and "members-only"-plugins. The first one creates my users after successful authentication against my ldap and sets the accounts to "author" (except the said two accounts).
Any ideas?