Whenever I try to cast a vote on primary, I get a red popup saying:
An error has occurred - please retry your request.
I have already force-reloaded the site, but that did not help either. What am I doing wrong?
There's a bug with some localization code - a fix is going out right now.
Also, your vote was properly recorded; the bug only affected any optional display message that might have been sent back to the client.