We have received a few inquiries on 404 error messages for /api/v3/users/status after upgrading to Mattermost v3.3. The issue appears on the desktop app (client) due to the cache not properly re-validating upon load in some cases.
After refreshing the desktop app, the error messages should disappear.
A fix has been submitted by the desktop community to address the issue, and are currently investigating whether additional changes need to be made. They will be included in the next desktop release.