Thanks, @jlampel, for the helpfull tips concernning the preparation for a successfull normal map baking! Is the division by 128 for the calculation of the UV island normal baking margin based on the normal map resolution just a value found out by testing or can it be justified mathematically? I know that 128 can be expressed as the result of 2 raised to the power of 7, but maybe there's an explanation beeing a little bit more detailed.
I fairly sure it's just in the range of good values found by testing and that number is specifically convenient because a lot of common texture sizes are multiples of 128. I'd bet it also helps the result of downsampling to be a bit cleaner. More here: http://wiki.polycount.com/wiki/Edge_padding