I was able to upvote it; you just have to do it real quick. ;)
But jokes aside, I wasn't able to upvote once the rep comment box comes up after on-hover.
I would have upvoted it for you, but I can't as it was me what writ it :)
Go ahead and click the button to add reputation, the difference between rep and a mere vote is the presence of a comment. In other words, if you leave the comment blank, it's just a vote even if it comes from the "Vote & Comment" button.
And we'll look into making sure that the comment box actually shows up below the arrows too. ;)