Default [Q] Can't connect to Win7 share from Android, but fine from other Windows machines

I have several shares defined on my Windows 7 desktop which are accessible from all other Windows 7 machines on my network. Until recently those shares were all accessible from my Android devices using ES File Explorer or AndSMB (Nexus 10, Nexus 7, Galaxy S4). Today I get this error when I try to connect to any of these shares from ANY of my Android devices:
Login fails. This may be caused by:
- WLAN is off
- SMB Server is out of network scope
- The IP is invalid
- Blocked by the firewall
- SMB service is off.

I connect to the server using my usual id/password (which is an Admin account, and has full access to the shares). All of the shares are listed correctly, but I get the error above when I try to connect to them.

Up to now I have been connecting to these same shares without any problems, using the IP address and my login credentials. I even tried creating a brand new share, will full access permissions granted for my user id. This share is also listed in ES File Explorer, but once again I get the error above if I try to connect to it.

I also have a Windows 2008 Server on my network. The Android devices have no problems accessing the shares on that machine. It's just this one Windows 7 machine that is the problem (but only for Android devices - other desktops on the LAN can access the shares no problem.

I've tried disabling AVG antivirus, the Windows firewall is disabled, and there are no other third party firewalls. Any ideas??