Slash commands not working for some users

Some users cannot execute slash commands for plugins

Steps to reproduce

Server: Mattermost 5.21
All users of a team are configured as either Team admins or System admins. Several plugins are installed. Server has been restarted.

Entering Slash commands for certain plugins don’t work for SOME users (like me System admin role)
eg. /standup, /standup report, /todo list, /todo add
Other commands work: /jira subscribe, /echo

Expected behavior

  1. Login into Mattermost
  2. Enter a slash command like /standup, /todo, /autolink
  3. Command is execute & result is shown

Observed behavior

Nothing happens. Server log is not logging anything. Nginx log is not logging either. This does NOT apply to all users in the team.
The slash commands are send to the server & correctly processed (Browser console), but the response is not displayed in the client.

eg. for /todo list the response is:
{“response_type”:“ephemeral”,“text”:“To Do List:\n\nNothing to do!”,“username”:“todo”,“channel_id”:"",“icon_url”:"",“type”:"",“props”:null,“goto_location”:"",“trigger_id”:“eswkjpbufjym8jjutayofkzo5o”,“skip_slack_parsing”:false,“attachments”:null,“extra_responses”:null}

Tested with desktop app windows 4.4.0 & chrome browser

you can enable debug logging to gather more information about what’s going on.

The slash command output suggests that there are no todo list entries.

did some more digging…
It seems to be related to our firewall. If I login via VPN and start a RDP session on my office PC everything is fine, where as using an app or browser via pure VPN or even without doesn’t work.
The server doesn’t show a problem & delivers the payload correctly but the client(s) don’t display it.
I’ll check the firewall log on Monday with my admin & post an update.
Thanks for now