@suthagar
Instead of downgrading to 3.0, first try this (this is what I had to do to make it work).
1) Take note of each of your file-shares actual location on the file system. Mine were all at /home/samba/shares.
2) Remove all existing file shares at Zentyal Web Interface > File Sharing > General > Shares. This will not remove the files from the files system; it will only remove the Zentyal samba shares of those folders.
3) Save changes with these file shares removed, and the save will actually succeed this time.
4) Remake each file share. When I did this, I accidentally put the entire path in at first (/home/samba/shares/FolderName), but then, after realizing this, I deleted my file shares again, and this time only put the folder name (FolderName), and Zentyal automatically knew the "/home/samba/shares" part.
5) From the command line, recursively change the permissions of each folder:
sudo chown -R sync:'__USERS__' /location/of/shared/folder/
Lastly, for some reason, when you try to control the access of share folders using: Zentyal Web Interface > File Sharing > General > Shares, if you add a group it won't honor it. You have to add each individual user.
This is clearly a bug, and one that is so ugly I'm sure they're working on this.
I didn't downgrade back to 3.0 myself. I've made the painful changes I've mentioned above and in hopes that an upcoming bug fix will make group access controls work again.
Similarly:
This freenas/zentyal user is having similar problems with Zentyal Group authentication:
https://bugs.freenas.org/issues/3276