I had the same issue when upgrading from 3.8 to 3.9 Team Edition. Now i am stuck on 3.8
By using Apache i didnt have to opportunity to Change this one (* below) according to the changelog, as it is only described for nginx…
Could that possibly be related?
Do you have a reference how that below mentioned line should be like in Apache?
(*) https://docs.mattermost.com/administration/changelog.html
• If you’re using NGINX as a proxy for the Mattermost Server, replace the location /api/v3/users/websocket { line with location ~ /api/v[0-9]+/(users/)?websocket$ { in the /etc/nginx/sites-available/mattermost NGINX configuration file.