FYI: Upon experiencing this intermittently for some compound search terms:
[phpBB Debug] PHP Notice: in file [ROOT]/includes/functions_content.php on line 1542: Undefined variable: profile_url
It looks like my login session may have somehow become forgotten since I was prompted to login in order to reply to this thread.
As such, this error string could be a mere side effect, rather than a problem unto itself.
This is still speculative. I have not fully tested this assertion.