There's currently a known issue with Cornerstone where it gets stuck loading or stopped by the error message when the page has lots of elements on some servers. It looks like you're having the same issue. The currently solution would be to click "Try Again" or reload the page several times until Cornerstone loads all elements. We understand it's not ideal. A performance improvement might be included in one of the succeeding releases which might fix the issue.
For now, you could also try increasing the max_execution_time. Here's a guide.