I am getting closer 
I found out that a leading slash is not good when adding the share. I used this because it looks like this in Esxi:
/filepool01/backup/backup-vmxx
but I needed to enter just
filepool01/backup/backup-vmxx, then the folders of the share are listed.
Maybe it would be a good idea to have a sample input displayed under the input fields so there is no confusion about that.
But the strange thing is: I only see the first level of folders, and cannot browse the contents of the folders, they all have 0 bytes. When connecting to this very same share with Esxi, I can browse all the contents there. Both clients have the same access rights - readonly - and when connecting I enter the same strings (except for that leading slash)
### edit
ok, that was too optimistic: restarting OMV seems to break that share: after a restart, there are no contents displayed in the shared folder, not even the 0 byte files.