Looks like I was a bit too quickly with opening this topic, just found out that it works when I add the permission “Write access to all hosts and folders”.
Maybe this also helps others having a similar issue.
Also maybe something for the checkmk devs to check if there is a clearer way to indicate the issue without having to dig into the logs for a strange 500 error. Sounds like something that 403 would be appropriate as a response.