Have you tried re-generating and installing the video modes in VMMaker after clicking EDID emulation?
Otherwise it *might* be because your adapter, or your video card, or UMSA, or SCART cable is not passing sync correctly. Or, to put it another way, not being fed sync according to it's expectations. Too many variables to know for sure, and I don't have an UMSA for reference.
You can test for presence of sync voltage at VGA pins 13,14 (H, V) with a DMM (should see something close to 3.5 to 5 volts).
It might work better with composite sync enabled via VMMaker. See if that makes a difference.
I know UMSA produces composite sync output, but does the UMSA accept both composite sync and separate H+V sync inputs? Is there an issue with the DVI-I to VGA adapter somehow so that it only passes composite sync?
Exactly what SCART cabling are you using and does it pass the full RGBs?