I'm not saying that it's not an issue with anyone else. It's just that unless I know if a response is coming back bad or something else is going wrong, I don't really have any idea if it matches any other problems that anyone else might be having.
Generally the flow is this: Your app on your server makes some calls to the SDK. The SDK composes a request to our API and then sends that on to us. We send back a response that's (hopefully) in a format that the SDK can understand. The SDK responds back to your app the status of the request. Your app then works with the web server to respond back to the browser.
If anything breaks along those lines, the web server's not going to get the information it was expecting to respond to the browser, and so it's going to throw up the 500 error. We could be sending back an incorrect response that the SDK doesn't understand, so the SDK falls down. Or, the SDK gets a correct response but has some other condition that causes it to break. Or, any of those other steps could be failing.
There have been other reports on these forums of developers having similar sounding problems related to the path the request is taking, and possibly getting an error response returned by a CDN in that path. Without seeing the actual transaction request and response ourselves, I have no idea if this is the same case or not.
If you'd like, you can contact support who can open a case for you. They can possibly investigate further, although there's not much to go on in your specific case. Please be prepared to provide a traceroute from your server to api.authorize.net.