I noticed that I can not show the onboard computer anymore.
After some experimenting, I finally figured out that it is only shown when the
Minetest window is fairly big / maximized.
This is probably not what it should be...
I'm unable to open the Onboard Computer or the Wagon Properties too. However in my case it didn't seem to depend on the window size (I tried several sizes/maximized the window).
Only some very few times I seem to be lucky and the respective formspec will open, unfortunately I have no idea what might be causing this behaviour.