Huh, that's interesting. Going through the backtrace, it looks like
Preprocessor_DOM::preprocessToObj is returning null when it really
should never do that.
For debugging, can you add the following at the end of the file
/includes/parser/Preprocessor_DOM.php (just before 'return $obj;'):
if ( !$obj ) {
throw new MWException( "No result for XML: $xml" );
}
And report the new error message and backtrace you should get?
--
Bartosz Dziewoński