Although there was a two way trust defined between all related forests (and domains), we were facing the issue that users from 1 domain did not get any search result (while users from all other domains did get)
We checked the trusts again, made sure DNS was working, checked the permissions within the Search application, created new “test” content sources, did full crawls, checked the application pool accounts but ended up nowhere until….
We changed the diagnostic level to verbose and checked the logs..
In the logs we saw the below error:
The solution for this was to:
So the reason it was not working for users from that one domain was because that domain was on a Windows 2003 level and the Search Service account we were using was not added to the groups mentioned above.