An authentication error occurred. See the VNC Server error log for details.

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This article applies to VNC Server running on Windows only.

The 2 most common causes for this error, and how to solve them, are shown below.

The Netlogon Windows Service is not running

Check that the Netlogon service is running (using Services.msc for example) and set to Automatic for startup type.

The user you are authenticating as has not been granted the required logon types

VNC Server authenticates uses by using the Network logon type. If the user you are authenticating as is not allowed to log on to the computer with a Network logon, you will see this error. To resolve this, there are 2 options:

1) Check that the user (or group(s) they belong to) is not being blocked in Group Policy, and amend the policy as appropriate. The relevant policies are:

Computer Configuration > Windows Settings > Security Settings > User Rights Assignment > Access this computer from the network
Computer Configuration > Windows Settings > Security Settings > User Rights Assignment > Allow log on locally
Computer Configuration > Windows Settings > Security Settings > User Rights Assignment > Deny access to this computer from the network
Computer Configuration > Windows Settings > Security Settings > User Rights Assignment > Deny log on locally
Computer Configuration > Windows Settings > Security Settings > Local Policies > User Rights Assignment

The "Allow" policies should be enabled, and "Deny" policies disabled for successful authentication.

2) Change the NtLogonAsInteractive parameter to True. This instructs VNC Server to perform an Interactive logon instead of a Network logon. To set this, open VNC Server's Options, Expert section and locate the parameter in the list. This option can also be set via Group Policy.

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