Nathan, you have mod_security or some other security module enabled on your server. Contact your hosting provider support team to disable it or configure properly. 

I disabled it using .htaccess Will that solve the problem do you think?

You can try to register a profile then, if you still have the error you need to contact hosting provider anyway and disable it or configure properly. 

Ok thank you for your advice, much appreciated 

I had my server host disable mod_security and problem still consists. Members can register but on the odd occasion im getting users complaining of this error

Nathan, most of the time hosting support team says they have disable the module, but they just configure it. Seems like they have not configured it properly or have not disabled it completely. This is the only reason I can think of. There's definitely some security settings which forbids user to access that page while joining the website.