Hi rasit ozdemir,
I think that this issue is occurred by the reCaptcha plugin in K2 component.
There is two choices you can use to resolve this issue:
+ change the recaptcha public key and recaptcha private key by accessing to the
www.google.com/recaptcha/admin/create or other site to get public key and private key. After getting them, you can change the recaptcha public key and recaptcha private key in K2 component (see the attached image).
+ disable the recaptcha in registration form in K2 component (see the attached image).
Thanks