Issue

Cannot access Synology NAS shares. Instead of showing a password prompt, Windows 10 throws following error message:

Open Folder
\\nas 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.

Resolution

The issue was resolved by manually adding NAS user login credentials to Windows Credential Manager.

• Go to Control Panel > User Accounts > Manage your credentials > Windows Credentials
  
• Click on Add a Windows credential and fill the credentials form.
  

October 2016
Synology DS413 (DSM 6.0.2)
Windows 10 Pro