Solved! Go to Solution.
Is the page you're trying to access on your local machine (localhost) or on an internal network? Optimizely won't be able to access any pages that aren't available publically. The reason you can view it when you use the "Pop Page Out" is that you're now using your own browser to view the page directly instead of Optimizely's.
I was able to get it working on a page that's not publicly available by creating a page with the following options:
Page Type: A set of URLs
Editor URL: http://localhost:3000
URL Pattern: localhost:3000
Match Type: exact match
Activation Mode: Immediately
By getting it to work, you mean that the experiment was running correctly for a page on localhost? That works, it's just that you won't be able to see any of those pages inside of the Optimizely editor - you have to navigate to the page directly.