Turns out my problem was being caused by the program Synergy (synergy-project.org/) which is used for sharing a mouse/keyboard between multiple computers.
Is there any way that a fix for this could be incorporated into DO? Or is this solely a problem with Synergy?
Many thanks for your help (and for creating the best file manager of all time).
I set up two virtual machines, one Win7 x64 and another Win8.1 x64, with Synergy 1.5.0 x64 running and connected between them, and Opus 11.5 x64 on each.
Drag and drop works normally for me.
I've tried on both the server and client machine, although in both cases only while using the machines directly. (I wasn't able to control one from the other using Synergy, as VMware won't let me send mouse input to one machine while viewing another. But Synergy was running, working and connected between the machines.)