however it seems to be working one day and not another.
That's the problem with the load balancing.
I'm assuming a URL will work as well as an IP address?
Well, the problem is, that the exceptions are IP address based (They can not be DNS based,
see previous discussions for details). It is possible to add the address in form of an URL, but
the exception is added as an IP address resolved from that URL. If the IP address changes
later, (eg. due to server load balancing), it will no more match the one in the exceptions list.