Solved.
When you aplly exception aggregation, which aggregates a lot of decimal values (money in my case) to one "total value", it seams there is a problem with internal calculation process. So, sum of +1.01 and -1.01 is not always equal to zero.
The result value, in fact, might be in a range from -0.00001 to +0.00001. Thus, check this result value for zero doesnt work. But check for a range -0.00001 <= X <= 0.00001 works fine.