membership invalid form submission

I have setup membership which seemed to work with a basic access level and subscription (no payments). It is being used in conjunction with appointments plugin. When you go to book now which is a protected page, it asks you to goto register if you wish to book online. When you click Sign On the pop up works but when you try to login or register as a new user the message "invalid form submission" appears. In safari if you refresh it logs in despite the above message. This has appeared quite suddenly and with no apparent reason. Some assistance please. The site is

For the record I am setting this up to simply permit appointments to charge zero for some clients. It will also be used for video tutorials at a later date but need a robust and working sign up.