Domain mapping "breaks" image editing

Ok... domain mapping has worked great for over a year. But within the last couple of days, if domain mapping is enabled (on any of 3 multi-site installs I've tested) - when we try to "edit" an image all that "pops up" is a blank white box.

You should know:
We've updated our ModSecurity to lock the server down more. I believe this may be related but my techs say since no errors or warnings are thrown it is probably not. They also tell me that this set up is wrong (because the A record of the domain does not point to the domain's own public_html root in CPanel (each account has their own CPanel because they have email at their domain etc).
You might have guessed, I run a linux box with WHM/CPanel.