Does the 404 error happen just after entering your account credentials, or aftter you click "Allow Access?" I can't say for sure without knowing more, but it sounds as though after you enter account credentials and click to authorize, you may be getting redirected properly to your redirect uri, but the call being made to request exchange the authorization code that is returned as a GET parameters with that redirect is being made to an invalid url.
The flow should be:
1) You app redirects to our authorization url, where you login and click "allow access"
2) Our server redirects browser back to your redirect uri with GET parameters, including authorization code
3) Your app redirects back to our token url, passing the authorization code back (among other parameters)
4) Our server validates the request, generates the token, and POSTs back to your redirect uri.
So, it sounds like step 3 is where you're probably getting the JBOSS 404, and that probably indicates this redirect isn't correct.
It should be to: https://oauth2.constantcontact.com/oauth2/oauth/token? with parameters appended. Let me know if this helps.
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