I run VST scans manually, and after installing another VST I ran a manual scan and the scanner crashed upon reaching Melodyne. Just checked and the Region FX for Melodyne crashed SONAR as well.
 
I tried to install M4's trial, then re-installed Editor 2 over this (after simply uninstalling both), and works fine in standalone mode, but there is an "M4 remnant" on the loose somewhere affecting both SONAR's VST scanner as well as the Region FX. AFAICT the vst3 file was not renamed, and I just ran the Editor 2 installation in "repair" mode to no avail.
 
I remember seeing comments related to this, but cannot find the solution readily. Anyone recall what the "fix" is?

The easiest solution actually worked... I simply deleted the C:\Program Files\Common Files\Celemony folder and ran the Editor 2 installer again in "repair" mode and it recreated the directory (with 3 subfolders versus a single vst3 file which was there prior to deletion).