Keycloak gives me the error response invalid_token: Token verification failed.

When looking at where this error is thrown (here), it just doesn't really clarify for me what part of this "verification" could possibly cause this to occur.

Setup
I've setup Keycloak with docker compose, alongside with Traefik, to use OAuth2 for my Symfony 6 application.

The idea is as follows:

• User goes to symfony-app-dashboard.local/connect/keycloak in the application gets redirect to keycloak.development.local to login (with the correct realm).
• User logs in successfully and gets redirect back to symfony-app-dashboard.local/connect/keycloak/check to authenticate the user further.
• The exception invalid_token: Token verification failed is thrown (found in the WWW-Authenticate header when dumping the Guzzle response)

The container for the application has access to keycloak through http://keycloak:8080, which is also setup as proxy in the knpuniversity/oauth2-client-bundle configuration.

For the debug log, see this issue.

Question
Can someone explain me in easy to understand terms what causes keycloak to return invalid_token: Token verification failed?