That took care of the problem for me. Thanks, TT120. Still, this is going to be a nuisance for all future users - the format I used is the standard format for Twitter urls. Why not just lose the validator and save everybody some time and trouble? Answering the same question over and over doesn't see...

I just tried submitting the url for my new Twitter https://twitter.com/theme_camps in that form, as http://twitter.com/theme_camps and then tried just submitting theme_camps In all cases I got the same error message: "The Twitter website address has to be valid. For example http://www.twitter.com/NA...