

Update: finally I managed to get it to turn, with a 'Mole' wrench (Locking pliers) and now rather sore handsEven when I apply great strength, it is useless: does not move a bit. To apply this much force only results in breaking the hexagonal 'hole' in the bolt, thus losing grip with the hexagonal wrench.
For a "second opinion", I've taken the A-bike to a 'regular' bicycle shop. They tried it with both a hexagonal wrench and a regular wrench/spanner, but with no result.
Now I don't know what to do...Any ideas?

But the nut/ring rotates with it. And it still won't come out. So there's yet another challenge for me to overcome...