Thank you very much, Wrong_Way, this was the right way
However this was, in my case, only the first part of the solution. BUT it was a necessary part, as I realised then that the stuck part wasn’t located in the head itself, but in the part above (where the filament is pushed into the head).
Removing the head like you explained, and noticing the head fluently let the filament flow through, I then saw that the stuck part of filament was still there, i.e. in the upper part. I could then see that this one had been blocking all the rest. Consequently, I could remove that part of filament, and the printer is working again.
Thanks to your input, I not only got too know my printer a little better, but I also have a working printer again
(I should maybe have followed my son more closely while he had been assembling the printer, some year ago)