Can you rebuild a Sram gear hub?
I'm very skeptical that an internal fault in the hub is causing the external mounting of it to loosen up... unless you are experiencing frequent total rear wheel lockup and skidding, maybe. I have a SRAM P5 hub that I built into an old mountain bike myself. Originally I ignored SRAM's torque instructions for the rear axle nuts and just tightened them by judgement. I have rebuilt car engines this way in the past, with no problems. But after having my rear wheel loosen up, I got out the SRAM specs and a torque wrench. Let me tell you, they specify cranking those nuts A LOT tighter than I would have thought, or dared to do without the torque wrench.

I'm also skeptical that nobody in Denmark fixes internal-gear hubs. Never been to Denmark, but thought bikes with such hubs were very popular out that way.