Is this something related to my server PHP
configuration?
Is strange because I get this errors in my wikis with Semantic Mediawiki
(1.2.6) but not in wikis without this extension.
Special:AllMessages can be an extremely long page. If Semantic
Mediawiki has lots of additional messages (which it probably does),
then that would make it even longer. The page may just be too long for
your server to cope with. Some way to show the messages once letter of
the alphabet at a time, or something similar, might be quite good. My
browser struggles with the AllMessages page for the English Wikipedia,
so I expect a lot of people would benefit from breaking it up.