Hallo, das ist ein Bug in der foundry app. Es gab einen hotfix für 0.8.9. dazu muss man die foundry app neu installieren (in-App Update reicht nicht).
Für mehr Kontext, hier die offizielle Ankündigung von dem Hotfix:
Hello @everyone! In addition to yesterday's release of Version 9, Development 1, our dev team took the time to backport a little something to the benefit of community developers and creators who host their content through their own third-party sites.
**0.8.9 Re-Build with new Electron Version**
--------------------------------------------
We have re-built the 0.8.9 release of the software to use a new Electron version (12.2.2) which resolves the issue with Electron's handling of LetsEncrypt SSL certificates. There are no code changes to the Foundry VTT software in this update, but a re-installation of the software will obtain the new Electron version if you are experiencing issues with installing modules hosted by developers who are using LE certificates for their module hosting.
If you choose to re-install the software to obtain this new Electron version we recommend that you first back-up your user data as a precaution and then perform a re-installation of the software using the version downloaded from the website at <https://foundryvtt.com/me/licenses>.