I have this MU Domain Mapping plugin set up. I parked my 1st domain x.com and setup using instructions here. It works.
I added an addon domain x.net pointing to /public_html/. I added it to the same blog as x.com. It loads into x.net/cgi-sys/defaultwebpage.cgi which is "403 Forbidden "
If I set x.net as primary, then all domains fail.
I added another addon domain y.com as a new blog. I setup the new blog in MU Domain Mapping. The y.com doesn't work either.