It could be an error returned from the broker, such as invalid queue name for example. Try running curl with verbose -v option. Feel free to send me the output in a private message and I can take a look.
In terms of the other requests, there is a tool for resending queue messages, such as from a DMQ, you can try here
For browsing queues and viewing message payloads, I used SDKPerf with options: -qb -md -q