Thread:Project:Support desk/Error when trying to watch pages/reply (2)

Strange error, I have not seen that one before. But: I found a patch for it: See here. However, MediaWiki 1.18 is older and no longer supported. You could use MediaWiki 1.20 instead of 1.18. The patch, which I linked you is part of MediaWiki since version 1.19, so the problem will be solved with an upgrade to 1.20 as well.

I would recommend doing an update. See Upgrading for everything you need to know. :-)