@m.traeumner My bad I misunderstood the instructions.
By the way I tried today the Bluemind (https://download.bluemind.net/bm-download/ ), it is an opensource however the calendar sync and activesync are for subscribers only (paid services)
It is nice, not complicated very classic in the offering. Weak when it comes to LDAP and AD.
On the other hand I suggest that you try the iRedmail ( http://www.iredmail.org/ )
It is opensource, combines SoGo and roundcube. full fledged. I failed to install on Nethserver however I was able to load it on a separate Linux machine.
I am yet to try Horde prior to comment on it as well.