I love the Nextcloud functionality which pre-installs with Mail-In-A-Box but their business model charges license fees to use for enterprise. When I try disconnecting Nextcloud functionality it messes up Mail in a box via Exchange connected apps.
Does anyone know a truly open-source, open license calendar server that I can use with Mail-in-a-box? Or how to unlock calendar from MAIB without using Nextcloud?
Are you sure about that? I just downloaded the server version directly from their site and the only license included is AGPL 3.0.
As far as removing Nextcloud or anything else, MiaB is basically just the whole package. If you want to change it, you are on your own to make it work.
Yes, they offer a paid service. This is a common business model for open source companies, including the maintainer of Dovecot and probably other software in MiaB, but it does not change the license that ships with the software.
Apparently they use GitHub (I never looked before), if you would like to review the license and code there:
The subscriptions are for the professional version which offers additional features and integrations. The source code is open though and you can use it for what you like. This is pretty much like most open source projects there days. You either spend the time or the money.