to pitruchio:

You are right! I have copied the gmapsupp.img from Navicom Roads of Russia TOPO - Version 6.09 to sd-card/garmin/ and did run the MapUpdater616All_1281.exe.

This did update the gmapsupp.img in the sd-card, the updated img is decrypted!

Problem solved, nevertehless it would be interesting how fxor works...