Hello, yes, I can see your reply!
I have personally came to the conclusion that this error is most likely due to the configuration as a whole of your Mattermost server, because the URL of the web server should not have a port included with it.
This will require several steps to remedy, which include modifying your Mattermost config.json file, as well as changing the web server configuration file for your Mattermost instance to include a reverse proxy, to route correctly.
In addition, I would be more than happy to assist in obtaining a SSL certificate so that you can serve your Mattermost instance over HTTPS, and in addition to that, I believe that I can assist you in properly configuring your VHOST so that you are taken to the correct domain when you visit the URL, regardless of what port is given to the domain.
However, this is somewhat complex, and I am not sure of your experience level working with the Linux command line, is this something that your familiar with?