Hi,
When I submit a comment the site redirect me to a blank page where is written {"message":"Comment added and waiting for approval.","cssClass":"k2FormLogSuccess"}

This is not convenient! I think there is an error, how to fix it?

Best regards!

I manages this in k2 parameters -> CSS & JavaScript Settings, I changed jQuery library handling :)

This was indication of a JS error, if that solved it (eg: jQuery was being loaded twice) I guess you found the solution.