Superior Drummer 3 plug-in in Reaper throws ""Audio engine inactive" error

[nroozebo]

Hi! Reaper recognizes SD3 after installation and linking it into Reaper, and I can invoke it on a track. But when opening the plugin window, it shows "Audio engine inactive, check your audio device" in its bottom right corner, and obviously is not able to play any sound. This is while my standalone SD3 on same machine runs and plays without a problem.

Any suggestions as to where I should look to fix this?

Reaper: v5.965/x64 on Windows 7
SD3: Superior Drummer 3 plug-in version 3.1.2 (build 20164) [64-bit VST]
Standalone host version 3.1.2 (build 20164)

Plugin path: C:\Program Files\Common Files\VST2\Toontrack

Audio output: Focusrite Scarlett 18i8

Thank you!
Nic (new Reaper user)