When using 2 different screens on different resolution in Win10, it may cause problems in programs using floating windows. Reaper is in this category. I was using a Dell U2311 at 1920x1080 and an old HP f1713 at 1280x 1024.
Solution: switch on Multimonitor v2 HiDPI awareness mode in Reaper. No more crashes since 
Posted text below about version 2.1 (no response…) but also version 2.1.1 is not stable in both vst2 and vst3 version. I keeps on crashing Reaper…
“Nice new features, however on my system Scaler is far from stabile. It crashes all the time. Just load an instance in Reaper and wait a few minutes and …gone…even without playing
Using Scaler 2.1 vst2, Reaper X64 v6.13 - windows 10, Focusrite Clarret 4, ASIO, 96.000, 256, i7 8700K, 32gb mem. Edit: Vsti3 version seems okay”