This is another matter. If you're setting permissions to allow specific users, access to specific shares, some extra work will need to be done.
Read over this post and see if it helps.
I' ve reinstalled all OMV OS and following the instructions of your post and he result is:
On a public share folder I' ve complete access and write/read/erase files inside
On a private share folder I get the little windows asking me User and Passw but I cannot go inside. I' ve tried with my PC bios name (CORE-86E45CBFFC) and my PC password. User and pw of others user of the OMV user group and crossing user/pw in infinite combinations.
Nothing to do
Update: I' ve tried NOW by my smartphone using the name/pw of my PC and it work.