Registration takes user to search results page!

I'm working on an existing Elgg site that I have upgraded from v1.4 to v1.7 (could not do 1.8 because of existing theme and plugin incompatibility). Now, when a new user tries to register they are taken to /pg/search/?tag=Search no matter what they fill in on the registration form. I have the siteaccess as well as search plugins, but even with these disbaled I have the same problem. Has anyone encountered this before? I would love ot know how to fix it as user's are currently unable to register. I have my changes on the dev site only, so cannot post the link for you unfortunately.

There are a number of other plugins installed, but having disabled them one by one I still get the same problem. If I disable the theme then the registration form returns an error on the email address input field every time no matter what is put in there and the user is not taken to any other page. The problem is likely in the theme somwhere and is probably a result of it not being entirely compatible with v1.7, but I can't find the answer. Any help would be greatly appreciated.