After a few days of disabling plugins, trying different themes and eliminating variables, I finally found out what my problem is.
If I don't have the Mapping Plugin set to 'Re-direct admin pages to the original domain'.. the insert buttons inside of the backend don't work. They just open up a blank box.
So if I am on a site that I mapped out and then go into a post or page, the (insert) hyperlinks, image, table, gallery, symbols, etc, don't work any more. I click on them and they bring up the box but it's empty.
To clarify, this happen when I am using a domain that is totally separate from the network's original domain.
Wondering if this is a bug new with 3.0 or a known issue. Any direction on this would be extremely helpful, appreciate the help!