OK... this is a server validation error on the radio buttons. It does appear you have a value specified, so the problem must be in the server validation code itself.
Copy and paste the server validation code to a reply and I'll try to spot the problem from there... actually I seem to remember a bug being reported on this issue.
In the server validation code make sure the radio button is referred to as:
You may have to remove:
if it is appended to the name. That should resolve the problem.