Hi,
quick update: I have temporarily set role of the grafana user to Administrator with the same behavior (same log entry in web.log and error message in grafana configuration page). When setting it back to the custom grafana role and setting the debugging level of “authentication” to debug, the connection suddenly started working successfully.
What I have done as well is mutually pinged the checkmk and grafana servers from inside the containers to check if the docker network has been configured correctly and I have recreated authentication secrets a couple of times.
Now I do not know what the cause has been for the problem (special characters in the secrets by accident?), but I guess I do not need further help now that things are working. At least the permissions mentioned in the documentation seem to be correct.
Thanks!