I think we managed to solve the problem.
My nephew’s computer is maybe a bit slow and needs more time to load the LSPDFR plug-in.
So we prolonged the timeout from 10000 ms (=10 s) to 60000 ms (=1 min), which is the maximum value.
For those who read this – you can find the timeout value in the Rage Plugin settings. You get to these by holding Shift immediately after start or by clicking on the round button with a gear sign on it (be quick while the loading screen remains visible).