Update (after a few monts investigations & testing): Unfortunately, The problem seems to be very difficult to replicate and without our current forum software and the database engine version.
Quick workaround, until there's a real fix: Search is now giving 100 results instead of 10.
What I've been working for the past year now, few hours every week, is mostly trying & testing migration of the whole Forum to a new platform. That will be huge, and I'll open it for testing by summer.