Regarding the rebuild not continuing, open the dev tools before starting the rebuild process, and look at the first request to assets/components/contentblocks/connector.php. That’s the request actually running the process, the others just poll for updates.
Perhaps the request comes back with a PHP error response for one reason or another (time limit? error in a snippet or plugin on a specific page?).
You can also try to find out what resource is causing the error by looking at the log in the console. What is the last one it processed, and which one would logically be the next? Can you save that resource normally if you try to edit it?
If you’d like us to take a look, please email email@example.com with details. Generally speaking that is also the better platform for anything urgent, as we don’t necessarily monitor the forums all times of day.