I first noticed it March 22; prior to then the names were visible.
Same thing in Firefox 19 and IE 9.
I recieved rep about a half week ago. But since Wednesday I'm having the same problem as I noted in the OP. Unable to mark helpful/unhelpful posts.
I got a reputation about a week ago, but still haven't been able to give anyone else one.
It's mainly a bummer because I want people to know they are recognized for adding great content, helpful and interesting, to MC.
Do you get any kind of an error message when you try to leave rep? Or does it just not go through? Have you tried logging out, clearing cookies and cashe/history and logging back in again? Sometimes this fixes little misc. random bugs......
After filling everything out.
I get a pop up that says. "Message from webpage" And inside the pop up it says "You must enter a reason for giving positive reputation" or "A comment must be left for negative reputation".
I've tried various things. When I earlier ran into this problem, I never figured it out, it just started working again. I'll keep messing with it.