I found the most conclusive evidence as to the symptom and cause here:
Basically, if an SMB server has any anonymous shares, it appears that the KB3189866 patch causes Windows not does not ask you for credentials and just fails. You can get a number of different error messages for this. In my case it was:
The error you may get is "\\SERVERNAME" is not accessible. You might not have
permission to use this network resource. Contact the administrator of this server
to find out if you have access permissions. A specified logon session does not
exist. It may already have been terminated
There are two workarounds, either:
- Uninstall KB3189866 (useful for proving it is the cause)
OR
Add credentials for that share into the Windows Credential Manager:
a. Go to Control Panel -> User Accounts -> Manage Your Credentials
-> Windows Credentials
b. For each server you wish to access, add in an entry (using "Add a Windows credential") with:
- Internet or network address: \\SERVERNAME
- User name: guest
- Password: guest