Evening Andreas,
Mine did exactly what yours has done, migrated towards the bolts.It was an easy fix.
All you have to do is move the reluctor ring so you can properly clean the mating surface where it should seat by removing any corrosion or road dirt to give you a nice clean surface.
Then try your best to clean the inside face of the ring if possible, I used isopropyl alcohol on all surfaces to remove any grease or left over residue from cleaning.
Use some stud locker on the surfaces and the tap the ring back into position rotating the driveshaft to ensure it is perfectly inline with the sensor and has no wobble as you rotate the driveshaft.
leave for the stud lock to go off, then enjoy blatting with a speedometer again.
I hope this sorts your problem.
Ralph