The K2 website is one of the most popular in the Joomla! community and thus it gets hit by lots of spammers.
Because we prefer to provide help to K2 users and not chase spam posts all the time (we're talking about human spam here, not bots et al), we recently had to take additional measures by deploying anti-spam checks using the StopForumSpam.com global database.
This means that from the thousand logins and signups we get per month, from time to time, legitimate users may get caught on as well.
So please either email us (use the site's contact forum) or send a tweet to us at @k2devs so we can whitelist and enable your registration.
If you tried to login using a social network account and got caught by our anti-spam system, try creating a standard Joomla! account. Or vise versa.