"invalid_request" when getting OAuth access token

This is a question I've seen asked by many people, and I haven't yet seen a good answer for it.

I'm trying to generate an OAuth access token using HTTP. As per the guide, I have copied the suggested URL and replaced it with my own redirect URI and Client ID

https://secure.na1.echosign.com/public/oauth?redirect_uri=[MyRedirectUri]&response_type=code&client_id=[MyClientId]&scope=agreement_send:account

But I still get this message when I copy the resulting URL into my browser:

Unable to authorize access because the client configuration is invalid: invalid_request

For the record:
- The redirect URI I supplied is public-facing (It's a server I administer)
- agreement_send:account is enabled in the OAuth configuration, as well as user_login:account.

- I have tried both agreement_send:account and user_login:account separately and together as "scope=user_login:account+agreement_send:account"

- The redirect_redirect is the same as the one specified in the OAuth configuration.

- The client_id is identical to the once given in the OAuth configuration.

- I have repeated these steps on three different API applications. I have tried various different configurations and scopes.
- I have tried this on multiple browsers.

- I have tried this with multiple different URIs as different subdirectories on the same server.

And yet I still get this same message every time, with no real indication of what's actually going wrong. Other posts seem to suggest that it's something wrong with the parameters in the URL, but I have copied the guide and suggestions on other posts exactly, and still no success. Please help.