AI OnAI Off
I found it. Looks like it is the EPiServer generic error page and that it is enabled by default. http://world.episerver.com/documentation/Items/Developers-Guide/EPiServer-CMS/75/Configuration/Configuring-episerver/
I turned if off by setting globalErrorHandling="Off" on the <applicationSettings> element, which allowed me to get the error information I needed to resolve the problem (which was that I was adding a non-serializable object to session and we had switched to StateServer mode).
I'm trying to do an AJAX partial page load. It works in my development environment just fine but deployed on our development server, it gets a 500 response. The strange part is that I can see while debugging that the action method my AJAX call posts to succeeds and returns the expected JSON response. Despite this, my browser still gets a 500 response with the following content:
An internet search suggests this is not a Google Chrome page, but an EPiServer page. Is it? If so how can I disable it or get it to give me details about the error? I have customErrors mode set to Off in the web.config. When my action method code is succeeding, I don't know where else to look.