Near midair collision between tow plane and orb

While towing gliders, descending through 2000 ft, a grey ball was encountered on opposite course. The object was at same altitude, approximately 50 meters away. I was in a shallow left turn setting up my approach, and the orb was spotted very close before merging to my right. When seeing it was not a bird I turned right (to end up on a crossing course) to be able to keep my eyes on the object. At the point of least distance between us I could see a matte grey color (seemingly) perfect spherical shape, and i followed the object as it drifted of to my right out of sight. The sighting lasted ca 4-5 seconds in total. It was mid to dark grey, and did not have a shiny appearance. My airspeed was 200kmh. The objects speed is impossible to estimate. The merge happened fast, and when turning to a perpendicular heading it was seen floating away at some speed, although possibly simply at the general wind speed. I did not see anything inconsistent with a party balloon, besides the seemingly perfectly spherical shape. The object was travelling northbound, pretty much from the direction of a town, and did not seem to ascend or descend. I got a clear view of the objects color and shape and can with certainty rule out bird, plane and drone, however not with equal certainty a balloon of some kind.