Problem with Theme Sandbox

Elgg 2.3.8

I happened to come across a problem with the Theme Sandbox.

Clicking on Components  brings a message:

Unexpected error while performing an AJAX call. Maybe the connection to the server is lost.

Clicking on Forms brings a message:

Ajax view 'theme_sandbox/components/tabs/ajax' was not registered

And in Icons - the Ajax Loader (.elgg-ajax-loader) does not respond.

All the rest links are OK, and the connection to the server is OK.

Is it a problem with my installation, or a bug of Elgg?