I seem to be unable to access the data highlighter in Google Webmaster Tools since I attempted to start a new highlight on a page.

Clicking the red Start Highlighting button to open the tagger did nothing, so I refreshed. Now, the page loads without the middle content section, then a few seconds later shows the following error:

Failed to load data, please try again later.

I can't get any of the middle section to load, even the list of current pages/page sets that have been highlighted—this error shows.

I thought it may be a Google service outage, but other sites' data highlighters work fine. It also seems coincidental that it stopped working after I attempted to start highlighting—I was able to list the existing pages and page sets fine before that, and still am able to access the service on other sites.

I've tried clearing browser data and have tried Google Chrome as well—same problem.

What's happened?