I know this post is old, but I had the same issue and finally came across a solution that worked for me.
I found the solution in this thread:
https://www.mobileread.com/forums/sh...d.php?t=120818
Steps: right-clicked the Calibre desktop app--> Properties-->Compatibility
Click to
uncheck Run as Administrator then click Apply.
Hopefully drag and drop now works for you :-)