I have cleared the cookies and cache. It gives me the same error message.
I use this to reach the ACP login. I very briefly see this. Then I get this error.
I tried duplicating the registration error to give you more info. I found the email, clicked on the link, but it told me I had already validated.

There is no confirmation upon clicking the link that anything happens. You get forum page with no forum, just the top ad and the bottom ad right together. No text saying "Thank you for validating" or similar text that most forums give you when you click the validation link. It is very confusing because it looks like
nothing is happening.
Anyway, it told me to try logging in, so I did. It lets me log in, but my member group is still "validating." It won't let me make any posts.
Edit: I have discovered it till only let me access the ACP as root admin, despite my other login's member group being allowed access to the ACP. I have manually approved the validating members so they can post, but why is clicking on the link not moving them to the member group?