@HvP Unfortunately this does nothing for me. All I can see is a little 0 m/s flashing up for a second when I create a new stage.
Other than that nothing happens.
I then docked a different engine part but that didn't fix anything either. That broke the dV calculator thing (does it have a name) even more and I couldn't add or remove stages anymore but I could fix the issue by reloading the save. Weird.
However, I rebuilt the entire craft in the vab and will take my dV and twr readouts from there now by adjusting the fuel level.
Let me hear if you find a different way to fix the problem and I will try it out, but this should work for now.
Thanks again and have a nice day.