Well, it seems that after phpBB upgrade the [url=https://root-forum.cern.ch/t/forum-problems/13756/1 words" and “at least 4 characters”[/url] problems are back.
Could you, please, again change the search word length from 4 to 2 and disable the common word algorithm (and maybe one can also make underscores “ordinary” characters in the new phpBB version).