Hi Tim and everyone,I could solve the problem myself:
The limiting factor in the GARMIN 600 is not only the number of stored routes(50).
If you connect the Garmin to the PC you can see, there exists a folder named "GPX".
Obviously there are stored all routes, ever transferred from SkyDemon. The number of gpx-files in this folder is very much greater, than the routes one can see under the menue item "Flight plans".
I deleted the content of the folder and everything works again.