Any chance that it's a spider or some other client that you're not expecting? Any non-JSON request to /xas/ will trigger this error, so this could be just about anything.
For instance, try surfing to link text and you'll get the same.
There's also a chance that you've misconfigured apache to send certain requests to /xas/ instead of / (where the static media should be hosted)
Hi Samet, we still have this issue in Mx4.5.1. Did you manage to solve the issue? If yes, can you tell me what you did? Cheers, Arjan