Can't confirm registration on forum?

I signed up for Into the Black, have confirmation of membership and a post approved, but when I try to link my account here Forum Wheel just brings up a popup saying there has been an error.

It doesn't give anymore details, like bad usename or email address so I'm not sure what the issue is.

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Further to the above, the error message reads:
Sorry, there was an error subscribing you to this site.

That's it.