Is the popup really a part of DOM, or is there a popup controller in the base SS install? We are having a lot of trouble hunting this down, and it is very weird:
If we create a new install, then the popup will work, until we do a ?flush=all then it no longer works. Whenever it doesn't work getting the source of the popup shows that the main jquery file from jsparty is not in the list of scripts to load.
Since it works, then stops, we started looking into caching, manually deleting SS caches, making sure apache wasn't caching, making sure to clear browser caches religiously, etc. Still no dice.
We are actively digging around in things, but we are feeling pretty lost on this. It is really weird to upgrade an existing site by using an svn switch, and it doesn't work, while creating a new site, and finishing the install by using the exact same svn switch works fine. Until you flush.
I tried doing a flush on your demo sites, and of course they survived just fine. Apparently 2.3.3 is just not for us or something. It's also weird as everytime we think of a new possibility, we can pretty easily discount it (reverse proxy issue, nope test server isn't behind the proxy, APC issue, nope test server doesn't use APC, etc, etc).