Keycloak + Loadbalancer throws unexpected error: 400 Bad Request

i have for development a loadbalancer which is in roundrobin mode and there are 2 of them, for developing only.
So i tried also to turn one server off so i will get everytime the same to check if i can generate the token and getting the user data. It worked well, until i turn the server2 on again.
I read something but do not know where to start. Is something wrong with my frontend-application or should i configure something with Keycloak?

As i said with one Server it worked and i got the userinfo data only when one more is on i get this error: 400 Bad Request - response: {"error":"invalid_grant","error_description":"Code not valid"}
Does anybody had same issue with a loadbalancer and can help me?