Ok. Native Windows solved the problem and the map works in Oregon 550t without problems. Maybe I should try WINE next time, instead of virtual Windows?

Yes, I have also been thinking about disk space. I have freed up upto 4.5GB of disk space (yes, the disk limit is too small :-S) in the virtual drive, but obviously it wasn't enough. I processed the image on the mapped main drive (which is large enough) but maybe the unlockers use temporary space on the system drive only...?