When you start a new torrent and you have already existing files on another drive.
The torrent is in stop state.
You choose new location and browse to the existing files.
You get the window asking if you should merge.
You cannot click any button, only cancel.

Workaround :
Rename directory in torrent so the directory doesn't exist anymore on destination.
Browse to destination location.
Rename the directory with same name and it checks the files inside.

Thanks for reporting this.
The Devs should be able to fix this in v3.22 which should be available very soon, probably within a few days.