The old Special:Allmessages allowed to access the in php and xml formats apart from the normal html.
This is now gone with the new Special:Allmessages.
The main usage of the xml export was probably pywikipediabot, which is now broken.
Version: 1.16.x
Severity: major
URL: http://test.wikipedia.org/w/index.php?title=Special:Allmessages&ot=xml