Hi @RbDev,
even without the subpath Mattermost has worked with Apache just fine (I guess once you reach a certain amount of users you will memory wise be happier with nginx or something even lighter).
After the 5.1 release I made a few tests with the Univention app I’m maintaining and it’s default apache configuration (you can find the template at https://stash.z-hub.io/projects/K4U/repos/mattermost/browse/configure_host#94-171, the stuff that proxies to 8778 you can ignore, that is our Web Meetings server). There it worked out of the box. This in the other hand means to proxy /mattermost, you still need to proxy /api as well.