OK. I figured what is going on. The problem is that the login for the K2 component was disabled. However, after enabling it, it started to cause another issue conflicting with Joomla’s own login when applying the ReCaptcha.
In order to resolve the first issue (Empty solution not allowed). it is necessary to enable K2 ReCaptcha (dont forget to add the private and public keys, as the ones provided will not work).
In order to resolve the second issue (conflict after enabling K2 ReCaptcha), one needs to disable Joomla’s Recaptch (globally and under User).
