The Mattermost log indicates a problem with plugin sandboxing. I have verified that user namespace sandboxing is available on the Ubuntu server.
Steps to reproduce
Mattermost version 4.8.1, Ubuntu 16.04 Server.
The problem is logged when the mattermost service starts up or plugin support is disabled and then re-enabled.
Expected behavior
Plugins run properly in user namespace sandbox.
Observed behavior
Log output:
[2018/04/17 17:32:32 EDT] [INFO] Loaded system translations for 'en' from '/opt/mattermost/i18n/en.json' [2018/04/17 17:32:32 EDT] [INFO] Server is initializing... [2018/04/17 17:32:32 EDT] [INFO] Pinging SQL master database [2018/04/17 17:32:32 EDT] [EROR] SMTP server settings do not appear to be configured properly err=Failed to authenticate on SMTP server details=unencrypted connection [2018/04/17 17:32:32 EDT] [INFO] Current version is 4.8.1 (4.8.1/Thu Apr 5 21:17:10 UTC 2018/6dc9fa885dfd5448fb6910bfb0cb4f1179139ff8/none) [2018/04/17 17:32:32 EDT] [INFO] Enterprise Enabled: false [2018/04/17 17:32:32 EDT] [INFO] Current working directory is /opt/mattermost [2018/04/17 17:32:32 EDT] [INFO] Loaded config file from /opt/mattermost/config/config.json [2018/04/17 17:32:32 EDT] [INFO] Able to write files to local storage. [2018/04/17 17:32:32 EDT] [INFO] Starting up plugins [2018/04/17 17:32:32 EDT] [WARN] unable to create user namespace: fork/exec /proc/self/exe: permission denied [2018/04/17 17:32:32 EDT] [WARN] plugin sandboxing is not supported. plugins will run with the same access level as the server [2018/04/17 17:32:32 EDT] [INFO] Starting Server... [2018/04/17 17:32:32 EDT] [INFO] Server is listening on [::]:443 [2018/04/17 17:32:32 EDT] [INFO] API version 3 is scheduled for deprecation. Please see https://api.mattermost.com for details. [2018/04/17 17:32:32 EDT] [INFO] Starting 8 websocket hubs [2018/04/17 17:32:32 EDT] [INFO] Starting workers [2018/04/17 17:32:32 EDT] [INFO] Starting schedulers.
What permissions do you have on /proc/<MMPID>/exe where MMPID is the Mattermost process ID? Also does the user running Mattermost own the mattermost/ directory and all its contents?