I was updating my site today and suddenly all the popups in Joomla failed to work. I get a blank popup and an Uncaught exception: Permission denied to get property Window.tinyMCE.

It turns out the problem has to do with a browser security measure, the Same Origin Policy.

The same origin policy prevents document or script loaded from one origin from getting or setting properties of a document from a different origin. The policy dates from Netscape Navigator 2.0.

I don’t know what exactly is happening under the hood but simply put, there are two addresses that can be used to access my site—www.unauthorised.org.nz or unauthorised.org.nz. As Joomla was setup using www.unauthorised.org.nz (as shown under Global Configuration > Server > Live Site), that is the address that must be used to maintain the site. I was being lazy and simply used unauthorised.org.nz; hence the problem.