Alright, the issue is now solved. Thanks again @Karthikk for the great help.
In case someone with a similar problem finds this, the solution was quite simple.
As a brief summary:
- There where multiple search portals inside document A pointing to document B
- A rems appeared in the document queue of B
- Deletion of those portals did not work
To fix it, the referenced document B was newly created as e.g. B_new and the content of B was transfered to B_new by cut & paste. Now B was deleted and after that the portals could be successfully removed.
Portals gone, queue clean, everything is fine again