Assistance Configuring OIDC using Keycloak

Hi, I’ve got Grist running on my homelab server as a docker container. I’m now trying to set up authentication using Keycloak.

I’ve followed the instructions in the help centre at OIDC - Grist Help Center

But when I click on the ‘Sign In’ button on my Grist instance I get redirected to Keycloak which errors and shows the message “Invalid parameter: redirect_uri”

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Can anyone help me work out what I’ve done wrong in my set up to cause this error to appear? Thanks in advance.