Search query (number of unreadpost at this point is zero!):

https://www.phpbb.com/community/search.php?&sr=topics&search_id=unreadposts&keywords=portal

Search found 12826 matches - but can't switch to page 2 etc. via pagination links
Debug message in top:

[phpBB Debug] PHP Warning: in file [ROOT]/includes/sphinxapi.php on line 706: assert(): Assertion failed

I'm not sure if it is a phpBB.com bug or au general pbpBB-Bug ...