As the filament cannot be pushed through to the extruder, I have realised that it has broken off at the back of the tube that comes from the AMS and leads into the screwed-on connecting part.
How can I detach the hose from this part?
The PTFE tube is inserted in a connector. At the entrance of the connector there is a collet around the connector. You need to push the collet towards the buffer (using a screwdriver or the 3D printed tool I posted). While holding the collet pushed in you can now retrct the PTFE tube from the connector.