I am not aware of any setting to turn this off.
Are you sure you are dragging into the open plan in Archicad (with no other dialogue boxes open - i.e. you are ready to work in the plan) and are not just dragging onto the Archicad button in your task bar?
Have you tried restarting your machine? - the old IT fallback when nothing else seems to work.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11