Let me put this in as an official answer and please mark it for others to use.
If you are on V8, 8.1, 8.2, 9 of Kentico CMS and tried to use Accessibility validation and getting error "Validation server is busy right now and was unable to respond in time. Please try to validate the code again." This is because achecker.ca switched from http to https only, and you will need to update the validation URL in one line of the CMS code.
In \CMS\CMSAdminControls\Validation\AccessibilityValidator.ascx.cs find this line
private const string DEFAULT_VALIDATOR_URL = "http://achecker.ca/checker/index.php";
Change http to https, and it should work.