I got the error Client secret validation failed for client when using a Litium app one day. The reason was that Litium apps are not built to be used in a multiple development environment scenario and another developer has reinstalled an app on his machine. That way the app got new service account-credentials, that my environment did not know about.
To fix this you need to uninstall the app in question and remove the folder containing the app information as explained in the Payment and shipping documentation.
Litium apps are single install only, when you have installed your app once that apps install-link will not work again. If needed just follow the steps below to clear the app for re-installation:
Stop the container of the app you want to clear
Open the folder where you kepp the docker-compose.yaml file from the docker-task
In the /data-subfolder of that folder you will find a direct-payment and a direct-shipment folder, delete the one you want to clear
Start the container again and the install link will work
Thanks to Nils at the Litium support for pointing this out to me.