@mahescho ok, but even if what is logged does not look like an error to you if may provide additional insights here. Or does nothing get logged at all? Since this happens in a web browser there may also be additional hints in the browser console.
Coming back to z-push-admin do these additionally selected folders show there? Can additional folders be added through z-push-admin?
Maybe it would make sense to follow this up with the Kopano support directly, so that someone could have a look at the system directly?