I'm also having this issue immediately after upgrading to 3.3.0. I noticed that I managed to make the 404 errors associated with my own IP address go away, but I'm not sure how. It might have been one of:
- Restarting Mattermost enough times
- Restarting my browser (chrome)
- Restarting nginx
When my IP stopped showing up in the logs with this 404 error, my browser logged into Mattermost stopped displaying the top-bar complaint about an unexpected 404.
I'm still seeing the message reported every few seconds in the log with the IP addresses that I assume are other users who are logged in.