License issue with Xceed.Grid.v3.9

License issue with Xceed.Grid.v3.9

we use a licensed version of the grid in our products, without any problems up to now.
Now, we got one customer having trouble with the grid, throwing the following exception:


Exception has been thrown by the target of an invocation.
The license key used to license this Xceed product is invalid.
Xceed.Grid.Licenser.LicenseKey must be set to a valid trial or registered license key
before using this product…

As mentioned, we bought the product and use it without problems on (lots of) other computers. The customer where the problems occur uses the same release of our software as many others.
The problem occurs on 2 computers in this one company, so I suspect a problem in the configuration of their Windows systems (Windows 7 with .NET framework 4.6.1 installed, I did the same on a test system without any problems).

Would you have an idea what to look after in order to solve the problem?