Fuel tank lid door does not open on 855 TDI

855 TDI 1997. After a few days with freezing temperatures I wanted to fill the
2/3 full tank. The door over the lid did not open. The car´s manual explained how to get access to the lever near the right back lower lights to manually open the lid, I will do it during daylight time. But I wonder whether that has to do with the low temperature? Or is it a faulty damper motor. I could not find info on the VADIS or Haynes manual how to get access to the damper motor. How is it accessible and how should I test electrically whether voltage supply and motor function are ok? Could it have to do with manually locking doors? Regards Franz47