Thanks for the excellent detail, @mkx-l! I’ve asked @aaronrothschild on our community server to see if he might be able to lend a hand here.
One thing I notice: your Mattermost server seems to be using a subpath (exampleserver.de/mattermost) – and I think our Zoom plugin currently has an issue with that per https://mattermost.atlassian.net/browse/MM-23411. Any chance that might be the issue?