This morning our server was no longer reachable. I searched for the reason, but the error messages when trying to start Mattermost were not helpful. At last, I found out that Ubuntu ran out of disk space and the config file for Mattermost was empty?! I made room on the drive and replaced the config file with a backup. And I changed mattermost.service to use the binary “mattermost” instead of “platform”.
Mattermost now starts without errors, but sends no data. The web browser window stays empty.
This is true for all Windows PCs. Then I tried it with my Samsung Galaxy S10+ - and it works!
I have no idea where to look for this error.