Hi - here's the reply from our developer:

"It just means that the site doesn't recognise the verification code, in
this case apparently because it has already been used and the account is
already verified (the user in question wouldn't be able to post to the forum
to ask the question if his account wasn't already verified).

There's no particularly good reason that it couldn't just let you use the
same code several times (even though you're not really changing anything
after the first time). I assume it hasn't come up until now because most
people click the link once and immediately delete the email. Everything to
do with logging in tends to start with very vague error messages until we
know we need more, because it avoids giving too much information to someone
trying to hack the site."

I hope that helps!