One of the threads of the mounts of the rear suspension was badly damaged. There was just not enough material left to fix it. Because there is not really a big load on the screw – it just prevents the rear shock of slipping off sideways – one number smaller would do the job as well.
After thinking of this problem for a long time, I tried to just file it down one step to 8mm, to cut a M8 thread on it. It’s far from perfect, but I’m very sure it will do the job. And it was easier as I thought. The old thread is a good guide, and after a bit of measuring with the sliding caliper it was surprising quickly done. Now I think about doing the same on the other side, to have the same kind of nut on both sides.