The forum search (/mod/forum/search.php) is passing an array instead of string to s() when searching using the "is tagged with" feature. The result is no results, and the following warning:

 Warning: htmlspecialchars() expects parameter 1 to be string, array given in {site_location}/lib/weblib.php on line 100

This does not appear to affect functionality (searching for a valid tag returns a result).