Since rMWbf69459d5011: Upgrade to jQuery v3, ApiSandbox seems to stall out when an API request results in an HTTP error (e.g. due to a PHP fatal error). Before that revision, it displayed the 'apisandbox-results-error' message in this situation.
Apparently the jQuery 3 changes to Deferred behavior caused it to be impossible to have a then() filter return this (or anything else) in order to avoid replacing the existing promise that's being resolved or rejected.