Cannot Connect to Windows Server Shares For years now, I have managed a mixed-device shop, with Windows 2008/2012 servers. Typically, connections are made using smb://share_name, users authenticate, and file shares are then selected. For whatever reason, as of today, this is no longer working for a subset of Macs.

I have tried the alternative cifs:// protocol, using IP instead of device name, connecting via Ethernet instead of wireless, using FQDNs instead of machine names, changing DNS addresses, etc. and I still cannot see my Windows shares. 

All Macs are running El Cap 10.11.5. The servers in question are Windows 2008 R2 and 2011 Home Server. All Macs and servers have been rebooted and are at their current SP/patch levels. 

Does anyone have a fix for this issue? 

EDIT: I am seeing the same problem in other networks connecting via afp://. I have no issues connecting to the same machines via RDP, or VNC. It's just a file sharing issue, and it's not relegated just to Windows servers.

EDIT 2: I have noticed also that there are no devices displaying under the Network sidebar tab, even though there are dozens of known devices on our network, and these devices are displaying fine on other Macs.
Try running this command in Terminal:

sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.smb.server.plist SigningRequired -bool false