New to OMV so I might be missing something, but I had a lot of trouble getting an AFP share to work, the server would appear in Finder but when I tried to connect it said "Connection Failed".
After messing around I found the solution: the Privileges on the Shared Folder allowed read/write for my Group but not my User. Once I explicitly gave read/write to my User it worked. And yes my User is a member of the Group.
It worked with the SMB share so I'm assuming that privilege inheritance isn't working in the current version of the netatalk plugin.
I couldn't find any messages in the System Logs that matched the timestamps, but again I could be missing something.
OMV: 3.0.96
netatalk: 3.2.10
Mac OS: 10.13.2