This is a wrong error message that may occur after an HMI failed to return the license after checking it. We have changed the way the HMI accesses the license in V5.9.1, so this error does not occur anymore.
With older HMIs, you have to reset the trusted storage of the local license server on the target system as @Düscha wrote.
Important: After deleting the folder from C:\NexeedLocal\Licenses, you need to restart the target system and run the licensing workflow afterwards.
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