Hi, did you ever find out anything about this error? I'm experiencing something similar. I have a pretty basic web mapping app at this point. The user draws one or more polygons on the map and then clicks a button which calls the geometry service to simplify the polys, reproject them and calculate the combined area of them. I'm seeing the same error as you intermittently.
Please let me know if you learn anything and I'll do the same.
Thanks,
Paul