This solved the issue with all of your symptoms.
I suddenly started experiencing this isue with a Linkstation on my network. I had attempted to pull Active Directory users and groups from my domain controller to use in the access restrictions of my Linkstation. I told my Mac user to un-mount and re-mount the drive using his AD credentials rather than the pre-AD ones used locally on his Mac.
After finding the Linkstation's AD access restrictions' capabilities to be an abismal failure, I just reverted to an open permissioned workgroup drive. After this point, no other users could write to the folders that my Mac user was creating on the shared drive.
I had him un-mount the drive and re-mount it using the local machine's username and password that he was using. Now we're back to normal :)