I've found a disassembly video here:
https://www.youtube.com/watch?v=3Z2rW64TKTU
Maybe the spring is broken.
EDIT: Some grease and a too short spring. I've cleaned the pen and slightly lengthened the spring and it's better. However, the functionality is still lesser than the Steadler Noris Jumbo. Too bad, because I like the extra length of the Onyx Boox Pen2. Maybe the rubber inside the tip of the eraser will have to be removed.