I think I might see the same problem.
I have first reported and documented my issue here: https://github.com/mattermost/platform/issues/4332 where user @jasonblais told me to post in this forum instead.
I am running gitlab and mattermost installed and configured as one with gitlab-omnibus on a single server with a single private IP-address with both the applications on different ports (gitlab 443 & mattermost 8443). After a bit of fiddling with the certificates I managed to make mattermost trust gitlab and can use both the applications in browsers and also the mattermost applications for windows & linux, but the android app will always show me this error:
We could not connect to the Mattermost server or the server is running an incompatible version.From reading the posts above this issue seems to be caused by the self signed certificate - user @januzi suggested importing the local CA used to sign the certificate (that needed to be imported into the servers trust store to make mattermost trust gitlab for authentication) also somewhere into the Android phone might help circumvent the issue - @januzi - Did you try it? Did it work? Do you have a link explaining how to do such a thing?
Anybody has any hints for me what I could try to workaround this problem? 