External timeout :(

It really sounds to me like you could use the base healthcheck (normal HTTPS).

 

 

Health Check Settings for Recieve String:

 

Displays the regular expression representing the text string that the monitor looks for in the returned resource. The most common receive expressions contain a text string that is included in an HTML file on your site. The text string can be regular text, HTML tags, or image names, and the associated operation is not case-sensitive.

 

 

Note: If you do not specify both a Send String and a Receive String, the monitor performs a simple service check and connect only.

 

 

Try removing your custom health check and using the default. Normally the send and recieve strings are only used when you are attempting to do some specific check (Example: Creating a test.html file on the system that will return "Healthy" when a get is performed against it. This would allow the systems personnel to change the file to say something other than healthy...and have the BigIP Automatically mark the server as down so that they could deploy we code or do maintenance on the server).