App install from marketplace not possible, error: NOT FOUND


If I try to install an app from marketplace the only thing happens is a top right red error message: not found. The server is connected and registered to cloud. Push notifications via cloud working fine.
The browser dev console shows following after click on install in marketplace:

Server Setup Information

  • Version of Rocket.Chat Server: 3.6.2
  • Operating System: Linux (Ubuntu 18.04 LTS)
  • Deployment Method: snap
  • Number of Running Instances: 1
  • NodeJS Version: 12.16.1
  • MongoDB Version: 3.6.14
  • Proxy: nginx

Whats going wrong?

Thanks Andreas