Strangely the problem is intermittent. If I use the pc to simulate the modbus values I get similar results. The meters only work intermittently, some of them seem to be more problematic than others. I just ran the program 15 times. One meter didn't work 5 of those times, another didn't work 6 of them, the other 9 were reliable.
This behavior sounds a bit different from what I described in the original post. I was not polling data for all 11 meters last night so it looked a little different to me.