I have the same problem here.
It works very well with the URL in the browser and in the desktop-app.
But the Android-App (2.18.1, build 53603 and build 6000536) show this error with the same URL.
Mattermost from docker:
Mattermost Version: 9.10.0
Database Schema Version: 121
Build Number: 9713988960
Database: postgres
The web-server is apache.
The error occurs on two different Andoid OS, one Lineage 21 (latest version) and the other is an two year non updated Android.
Any help is very much appreciated!