I'd hate to admit, but this one took me a few hours to figure out...
See this request?
"http://localhost:8080/jsonrpc/?request=<uriencodedrequest>"
Well, that's wrong. It's supposed to be:
http://localhost:8080/jsonrpc?request=<uriencodedrequest>
Can you spot the difference?
Yep, trailing slash! Damn it!
Oh well, back to it!