Account verification error
I don't recall the specific version/build and my search-fu is weak today, but this was a bug in Craft that should have been fixed in a semi-recent build.  Try updating to the latest and see if you still have the issue.

Update:

Every time we've been able to investigate this issue so far, it has been because of either .htaccess stripping querystring params behind the scenes (i.e. no [QSA] flag on the rewrite rule), or the equivalent in nginx-land.