Official comment
You can thank Zendesk for that due to the amount of spam we've been hit with recently.
That post isn't entirely accurate - here are the cookies needed to be cleared if you have problems:
*.gigya.com
*.zendesk.com
*.pioneerdj.com
You should have 3rd party cookies enabled on this site (and those domains listed above).
Some users have found certain browsers to be less problematic than others, and likewise, plugins which block traffic, or javascript can also be a reason for these issues.