Hi,
I’ve found out the cause of the issue. It doesn’t depend on Mattermost, or, better said, it’s a consequence of how the feature works.
Since the connection between Mattermost and Zoom uses one single account, the owner of this account must add all users to the Zoom Users section. This operation can be done only if you add a paying method to your account, although - apparently - you don’t need to have a paying plan to use it.
Once you add a user to the Account, then they will be able to launch a call from within Mattermost.
See screenshot: https://i.imgur.com/wisi4YO.png
And no, it was not related to 4.5 vs 4.6 version.