Hi,
A quick update!
I decided to take it apart and as soon as I did it was obvious what had happened. Someone changed tool with the spindle moving at some point and the return springs for the cylinder got crooked. After enough cycles some of them broke and got stuck so that the cylinder couldn't move past a certain point, I guess the last one that made it impossible to change tool just lodged itself in there the other day!
I'm going to get new springs and try to put it all back together! I had to desolder a few wires but it was not especially hard.