Hello
I have problem with K2 registration, after completing the registration form, there is statement: "no solution is allowed".
Before K2 installation, Joomla classic registration worked properly.
katarzynaderen.pl/rejestracja

Please help me,
Regards

This message is from a captcha/ antispam plugin.
Make sure the extension you are using is compatible with K2.

I am using default Joomla captcha plugin. What should I do now?

There is a conflict in your settings.
Did you try the "Enable reCaptcha on registration form" setting in K2 (advanced tab).
Also you should remove the default captcha and use K2's captcha settings.