our company just recently started to use mattermost. We have received an invitation to the server, registered, connected. Desktop app works, in browser also works, but whenever i try to connect by android app, it shows me “The certifcate for this server is invalid. You might be connecting to a server that is pretending…”
My teammates were able to connect by app, on IoS and Android.
I think you need to ensure that the AndroidOS trust chain trusts not only the certificate of the Mattermost server, but any and all intermediate CAs and the root CA.