curl: (56) SSL read: errno -5961

I think -5961 is the error code for connection reset, which is probably the default action when anything goes wrong in your traffic flow.

Do you have any iRule code attached to your virtual server?  You see connection reset a lot when an unhandled error happens in an iRule. 

If it is not too verbose, try looking in the /var/log/ltm file on the device itself.

For handling a specific error condition gracefully in an iRule, look into the TCL catch statement.