In case it helps Max locate the error...

I thought that '500 License Error' was odd - certainly uncommon. Turns out you only get it with http/1.1; in Firefox with http/2 I get '500 internal Server Error'. I can get 'License Error' if I disable http/2.

That's in the raw headers; the message on screen is '500 - Internal server error.' in both cases.

Two screenshots from Firefox here - http/2 at the top and 1.1 below:
http://www.cjoint.com/data/GKtp3JZGcwl_mudcat-500.png

I find a request on the heels of another is more likely to succeed - e.g. when (I'm about to) post this.