When using the advanced labs' cloud IDE, firewalls, extensions, or proxies can block connections necessary for the IDE to operate. In this case, the page will still load, but some aspects of the IDE will not. The run button may not work, or instances might fail to connect.
If you're still having trouble after refreshing the page, check your browser's console tab to see if any red errors, like a 403 or 404 error, are visible.
Firewall issues can be resolved by whitelisting the addresses below either with your firewall provider or your proxying service. The *.codingrooms.com addresses are recommended, but some firewalls may not allow for URLs with an *.
If extensions are causing the block, they can be disabled one-by-one to verify which one is causing the issue. Make sure to refresh the page after each extension is disabled.
Contact support if you're still having trouble after disabling extensions and whitelisting the connections above.