phpBB SEO Premod for phpBB3 support forum. This premodded version of phpBB3 includes the three different type of URL rewriting for phpBB3 by phpBB SEO. It comes with several other Search Engine Optimization mods installed.
Does it goes back to normal with original files ? Do you also edit language files (if so, make sure you also did the edits with an utf8 capable editor) ? Did you try to turn phpBB debug on (please report about any error if any showing up) ?