The obvious possibility here is they could be filtering your request server-side, possibly by checking your reported User-Agent. This you could spoof, but doing so might be against Terms of Service, and is likely so if they've bothered to filter for it.

Could you share the real-world URL, either in a response or via personal message? I don't recall ever seeing a URI with a slash in a query parameter, so having access to the real case would make proper debugging a heck of a lot easier.