Users unable to register

We've recently upgraded our forums to version 9.71 but I've just noticed that new users are unable to register.

I get an error message saying username taken or username under 2 characters. This happens when trying to register obviously unique names such as testtesttest565656 and the like.

I cannot see any settings in the admin that should be preventing users from registering - i.e suspend new registrations is set to 'No'.

Any ideas in case there is something obvious I am missing?

Thanks in advance.

Thanks - that's sorted the problem. Would never have thought of that in a million years!