Thanks @mythz
We’re not running IIS in a 32 bit OS so unfortunately that doesn’t help.
In our code we don’t access HttpCookieCollection and this is how we create the client. All webservice requests are via ServiceStack.
What’s suspicious is that the ArgumentOutOfRangeException always happens on that request path. And every time the query url doesn’t look like it has been encoded properly. Perhaps a red herring but how can I get ServiceStack to encode it properly?