400 Invalid redirect_uri error

I have one website working fine with the V3 API.  I am now setting up another and getting the 400 Invalid redirect_uri error.

On authorize I redirect to this URI: https://api.cc.email/v3/idfed?client_id={my_client_id}&response_type=code&redirect_uri=https://www.scarsdaleforum.com/System/constantContact/token&scope=account_read%2Baccount_update%2Bcontact_data%2Bcampaign_data 

This redirect returns 302 and comes back with a response header of Location: https://idfed.constantcontact.com/as//authorization.oauth2?pfidpadapterid=ctctOAuth2IdpAdapter&client_id={my_client_id}&redirect_uri=https://www.scarsdaleforum.com/System/constantContact/token&response_type=code&scope=account_read+account_update+contact_data+campaign_data&

Notice it has a trailing &.  Also notice that the + signs separating the scopes are not URL encoded.  When I passed just the plus signs in the original redirect URL,  the redirected URL returned did not have plus signs but spaces.  Then I encoded them and the redirect URL comes back with plus signs.

The browser redirects again and the above URL returns the 400 error.

Any ideas of why this is failing?

Thanks,

Bruce