Generic MQTT-Things cannot be displayed with OH 4.3 - Error 404

[MartinBachmannHD]

After the upgrade to the latest OH release 4.3.0 I cannot access from remote locations the 'Settings' -> 'Things' -> for any generic MQTT Object any more. No Code, No items, ... -> Screenshot
Access from home intranet (192.168.x.y) for generic MQTT works fine, so I assume something with the external access.
Access from remote to other Bindings (e.g. Network) also work fine.

When looking at the Chrome Dev Console I can see, that the request gets an 404 error (not found):
https://home.myopenhab.org/rest/actions/mqtt:topic:xyz
All the other requests work fine (200)