I recived this error while trying to login on the "sign in" page. The system said it is due to an error in WikiUserNew.php in the function "isValidName" at the return value.
So I googled and figured out that replacing the "/u" by "/U" solved the problem.
I hope that helps.
Best
-Tswaehn
Logged In: YES
user_id=13755
Originator: NO
Fixed in current CVS.
http://sourceforge.net/cvs/?group_id=6121
or http://phpwiki.sf.net/nightly/phpwiki.nightly.tar.gz