I'm not really a web programmer, so I can't say for sure what is the issue, but I'll ask our team. I tested a similar call with curl and it works fine:
curl --data-binary '{"jsonrpc":"2.0","id":"2","method":"sps_api.list_proposals", "params" : {"start":"block", "order_by":"by_creator","order_direction":"direction_ascending","limit":100,"status":"all"}}' http://sps-blocktrades-testnet-api.blocktrades.info:8090
One thing I noted is that sps_api is specified in the curl call. But beyond that, I can't say.
Interesting. Would really appreciate if you ask your team. Btw, what is the best way to keep contact with you in case I have some other questions on proposals?
I suppose best place to ask questions right now is in the github repo, probably by posting an issue/question there.