Android app not connecting via VPN

I am trying to access to a WebDAV endpoint from the Android Joplin app (v 1.0.312), via VPN (namely Wireguard).

  • the WebDAV endpoint is a docker container running on Ubuntu 18.04
  • I can access it from the same phone via Chrome (after logging in I see the files)

The error on the app is:

01-22T16:16:22,10,"""TypeError: Network request failed onerror@index.android.bundle:139:2655 value@index.android.bundle:124:1505 value@index.android.bundle:123:6095 value@index.android.bundle:123:2835 value@index.android.bundle:49:1280 value@index.android.bundle:28:3311 index.android.bundle:28:822 value@index.android.bundle:28:2565 value@index.android.bundle:28:794 value@[native code]""