The 406 HTTP response means that the server didn't understand or didn't like the "Accept" HTTP headers. You don't have to believe me, you can
look here. ARC is built on top of the Appcelerator Titanium runtime. The Titanium runtime implements a WebKit browser which is equivalent to Safari and Chrome in both capabilities and functionality. ARC itself is written in Javascript and uses jQuery to communicate with your server. It's pretty much the same code used in Akeeba Backup's back-end to run the backup.
As you can see, the only difference between a back-end and an ARC backup is the web client used. In ARC it's a WebKit browser. It sends a regular HTTP Accept header over an HTTP/1.1 request. Your server replies with an HTTP 406 status. It didn't like the request. The only time I have seen this behaviour is on Apache 1.3 servers. Maybe you can ask your host about it.
See, you may have 100 sites on a host, 99 working and one not. This is because hosts never have just one server machine. They have multiple. Each site may be hosted on a different machine. Each machine may have a slightly different setup. If unsure, don't think; ask your host. That's why you're paying them, right? ;)
Nicholas K. Dionysopoulos
Lead Developer and Director
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