Problems with Plex Home - no access to libraries for external users

Server Version#:v.1.42.2.10156-f737b826c and latest beta

Hi, I have been running the Plex Media Server for quite some time and finally bought a plexpass a bit over a year ago.
I am running the Media Server on a QNAP TS-451D2, updated it from time to time and for a while everything was fine.

Unfortunately I managed to accidently activate Plex Home while inviting my brother and from there my problems began:

After activating it suddently my other brother and a close friend, who I had formerly invited as external users with their own plex accounts could not see any libraries any more.
If I looked at the settings it still said that I had granted access to several libraries, same on their side.
Newly created and invited accounts have the very same problem.

After reading a bit I found several sources that blamed Plex Home and said that as long as Plex home were activated the non-Home users would not be able to access any libraries.

Unfortunately I cannot remove my own account from the Plex Home or delete it at all.
Only my account, which is the original server admin account remains in that Plex Home.

What am I doing wrong?