Membership 2 Pro not working - people can't register

If you go to:
and navigate to "Buyers" -> "Investors"
and try to register to access the Investors Area - you can't...
The same page just reloads with an empty red notice box above the registration form...

Can you help resolve this?

My client wanted to setup a free members area for investors to help capture emails and I thought that the plugin Membership 2 Pro would be perfect for this but can't get it to work correctly... Should I be using a different plugin for this?

I can grant access if needed.