Hi,
Interesting, I ran into a similar issue. I don't get many failures but the "Monster palette" color chooser editor does not show up.
I tried both an older release (4.0.11) and 4.1.5 - the latest at the time of writing and both had the same problem.
I could try on different PC-s but unfortunately the license will only let me to try another PC and obviously not several.
I also think it has to do with the .NET version, I already tried 4.5, 4.6 and 4.7 but to no avail. Still, I will try to install different versions to see if anything changes.