Ok, just did some more testing, and as my export program allways gave "-0.8403" (tax value = 19% as stated in the export file) and never worked, my manual attempts with a fast "-1.0000" allways work.
I tested some more low value changes and all failed, by everything lower then -1.000 works.
Another strange thing I noticed (tax still 19%):
0.8403 * tax = doesn't work
1.0000 * tax = 1.19
1.5000 * tax = 1.19
see: http://www.speelmans.com/Example.rar