No to pi-hole and no to the logs - checked each available log more than once: IPS, system logs, URL filter, and more.
BUT I finally found the issue - mad bad! I’m using a DNS blocker (similar to pi-hole) and obviously the automatically downloaded DNS list, with host names to be blocked, contains the host names of hbbtv.zdf.de and some more.
Don’t know why the author wants us/me to block them, but after whitelisting some hosts, and rebuilding the blocker list, the ZDF Mediathek now runs well!
Thanks for all your suggestions and tips!
Michael