I am new to this forum but want to say that I am experiencng the same issue with 5.3.1 - it has simply stopped working on my Macbook Air. I have deleted the app several times and re-installed from the App Store, and updated the Mac OS to make sure it was not incompatible. The Mattermost 5.3.1 simply will not l oad and open on the desk top. I really hope this can be fixed soon.