I leave this here because someone else may have the same Issue.

For a test i changed my logitech mouse to a "schwaiger" mouse and the issue disappeard.
Still though it would be interesting to know why.

When dragging an image (png) onto iclone like described in this video
https://magazine.reallusion.com/2024/10/30/using-ai-generated-images-as-3d-scenes-in-iclone/
nothing happens. The expected submenu does not appear at all, or only for a few milliseconds
when releasing the right mouse button. It works may be every 50th or 100th attempt.