Let's iron out the network setup, and how the workstation is logging in. Because I frequently see workstation log in using a "local account"...instead of a "domain account".
Workstation should be logging in with the format DOMAINNAME\USERNAME and password. Not a local user account as in WORKSTATIONAME\USERNAME
Workstation MUST use the IP of the DC for its DNS. Nothing else. I typically see the router doing DHCP for the LAN, creating havoc with proper active directory functionality since the router gives out itself for DNS or the ISPs DNS...which of course neither of which know anything about the AD on the server.
Server...MUST use IP of itself..for its DNS, nothing else.
DNS forwarders set in DNSMGMT.MSC
On the folder share...
SHARE permissions...set to EVERYONE, full control
Security tab (NTFS)...I set to Domain Users, Administrator, System, Domain Adminstrators. Peel back as necessary from here....to limit end users, security groups for example.