i have now noticed that the users got copied from the external sql database to the oc_users table (probably by some broken owncloud upgrade in the past). the principal admin acount never was on user_sql and could always log in. with this admin account in "users" i activated "show user backend" — all relevant accounts had "database" written there, instead of SQL.

i'm not running any extra authentication plugins. the i'm using postgresql as the database backend.