I have established a connection between QGIS and PostgreSQL. As long as the active user has the 'Super User'check-box enabled, all works well. From the moment I disable it only the Schema's remain visible. The layers are gone. I searched the Internet but can't find any solution.
PostgreSQL 11 / PostGIS 2.5 / QGIS 3.4.1 / Windows 10 (home edition)