There is a serious problem with new user registration and I can't find the solution anywhere!!!
Joomla 3.4.1 with latest K2 2.6.9
No matter what I tried I either get the "Empty solution not allowed" or if I disable Joomla's reCatcha and I only leave K2s then the registration goes thru EVEN if the reCatcha's field is left blank!!!!
If there is a CURRENT solution to this problem, please post it here step by step.
If you have any suggestions about anti-spam measures, please let me know, because I get a lot of bogus registrations constantly!

Regards
Ted

I just activated both, Akismet and StopForumSpam.com...
Absolutely no difference. In Login screen, it only displays the reCaptcha frame, and once again, if left blank it accepts registration!
Check out the K2 parameters screen dumps that I'm attaching.
In case you want to try it yourself, the site is www.radioactivefm.gr

Comments Page : http://imgur.com/DYFuu1N
Advanced Page: http://imgur.com/Tj14rFU

No replies whatsoever ...
In anycase I have found that User - K2 was unpublished in the Plugins and that's why it was letting users in without having to fill in the recpatcha field...
So thats fixed now.

Update to the problem...
As I said, both now Akismet and StopForumSpam.com... are "supposedly" active.
But still users bypass (somehow, I don't know how they do it!!!), the recaptcha thing... and not only that, lets indeed assume that they're humans and they type in the recaptcha... How do they bypass or NOT fail the email verification?! Are these emails indeed active (like This email address is being protected from spambots. You need JavaScript enabled to view it.)?! I really wonder... (thinking outloud here)
So to continue, the only thing (antispam measure) on the login screen is the recaptcha, should something else from Akismet or StopForumSpam.com also appear?! I don't know?!
But its obvious that these two don't do anything...

I 'm not sure about which K2 plugins should be enabled in order for those two to work... I have unpublished several due to incompatibilies with other stuff, so if you know what could possibly be wrong in this scenario, please ... do tell!!

Regards
Ted

It appears that they are real people, so you should manually enable users.

As for K2 Plugins, K2 needs all of it's plugins enabled in order to work properly.

I Have the same pb !!!

Do not double post.
This is a captcha conflict. Make sure that you only have one captcha enabled K2's or Joomla!'s plugin.

In this forum there are several threads about this similar issue and how to properly setup captcha in your site.