I think this should be filled as a bug, it will eventually result in low volume with users. A solution would be the custom scale integrated as a special Vista select in setup.
Ok, my solution to this: Use the following volume custom scale
0, 4095, 8191, 1638, 12287, 16383, 20479, 24575, 28671, 32767, 36863, 40959, 45055, 49151, 53247, 57343, 61439, 65535
Then it works. Other scales can get stuck, so I had to come up with this one, which suits me best. There are of course others that work.
My suspicion is that there are some sort of rounding issues, because with other scales Mediaportal reports a value as "lastknown" value which is not a part of my scale but one more or less. I suppose then it gets confused if the volume is suddenly at a value which is not part of the scale. But this is just a first suspicion, I didn't do any debugging.