Yesterday I followed a hilly route I had done before, with a Garmin speed sensor 2 installed (since long). The speed was indicated correctly on the 830, but the distance remained at 0. Similar for the total ascent: it remained at 0, although the altitude and gradient readings were correct. Obviously the 830 did receive the speed sensor data, and read the internal barometric sensor correctly, but the software did not update the distance and total ascent fields.
After about an hour I stopped the activity on the 830, reloaded the route and started riding again - problem gone.