Hi everyone,
If you haven't found the answer to your issue by web searching the error message plus "mattermost", then please use this link to post a new topic in the GitLab Mattermost forum for your troubleshooting issue.
This was recommended in the first message on this thread.
We've also been trying to support people who report issues on replies, and realize that it's really confusing for anyone to follow, so we're going to close this thread so that people create new topics that can be more easily supported.