@Ballebar
Yes, MS flag is 0 in IMG-file - this was the first I checked.
When starting BaseCamp the progression bar for Wanderkarte is red and it stays red till the message comes, that it is not a valid Garmin chart. BaseCamp works some seconds (~ half minute) for this.
I have many other charts on virtual floppy drive working with BaseCamp very well.
I will try it with other 'Wander- & Reitkarten' - maybe it works with 'Inselkarte' and report results here.