Hi,
I've been doing a little work with AXL so I'm no expert here. In my experience an error 500 relates to the request you are sending. If you don't send the request properly in the body, missing a tag, not passing required tags, it can cause this.
I use Postman to send different AXL requests until I narrow down exactly what I'm trying to send. It also helps to see the responses in clear text. Once I have this figured out I'll move on to coding the request properly. Maybe this method will help you figure out what is going on.