At the first battle when the defense phase started I could not put any of my 5 star common monsters in my defense. When attack started I could not enter the war at all, and even now it's the same thing. These 2 error messages pop up:
An error occurred. Try again.

Unknown error detected. Please try again.

Hi and welcome to forum :)

Did you send a support ticket as shown here
https://forums.socialpointgames.com/topic/30/how-to-report-game-problems-to-support