Hi,
in Mediawiki 1.21.1 I got the following error which I managed to fix according to
Fatal error: Call to undefined function wfLoadExtensionMessages() in C:\inetpub\mediawiki\extensions\S5SlideShow\S5SlideShow.php on line 123
For the fix remove (comment) the line wfLoadExtensionMessages('UserMagic'); in S5SlideShow.php
Works for me! Best regards, Max