I looked for this problem but couldn't find anything so I thought I'd give it a try here.
I am running OMV Release: 5.6.5-1
with support for Timemachine enabled.
Since the beginning (some earlier version of OMV V5) I had the computer I am typing right now doing it's backup via Timemachine (over WiFi).
It is running macos 10.14.x and the backup works perfect.
Last week I put an old iMac (Mid 2007) to life by installing the latest possible OSX which is 10.11 (El Capitan)
After everything was setup on this old iMac I tried to configure Timemachine but it won't work.
This iMac under OS 10.11 doesn't even see the Timemachine-Server (my OMV server)
Is there something with OMV V5.x and older Mac versions?
Thanks a lot in advance!