I am experiencing the same error, the exact same setup was followed on Amazon Linux 2 and was working with Keycloak, but when I attempted to integrate the 2 on a RHEL 7/8/CentOS7 system I get a 500 error.
I am not using a load balancer, using Nginx to force https; I tried to use LetsEncrypt to get a cert but the RHSM threw an error, and I was not able to login (at the momentt). I did not see any other errors, it appears that Keycloak is setting the session and attempting to exchange the token with GitLab but then I get this annoying 500 error. Any help would be greatly appreciated!