Unauthorized endpoint error even though the remote site has been added

Question

I'm getting the following error authenticating with the UN/PW oauth2 flow (I redacted the actual domain name):

Unauthorized endpoint, please check Setup->Security->Remote site settings. endpoint = https://CUSTOM_DOMAIN.cs32.my.salesforce.com/services/oauth2/token

I have added a remote site setting for this domain but the error is persisting. Has anyone else experienced this doing oauth2 callouts to orgs with custom login domains?

NOTE: this org does have this checkbox enabled Prevent login from https://test.salesforce.com.

EDIT:

    if (customer.thisOrg.domain_override__c!=null) {
        req.setEndpoint(customer.thisOrg.domain_override__c+'/services/oauth2/token');
    }
    else {
        //check to see if type is sandbox or production/developer
        if(customer.thisOrg.type__c == 'Sandbox'){
            req.setEndpoint('https://test.salesforce.com/services/oauth2/token');
        }
        else{
            req.setEndpoint('https://login.salesforce.com/services/oauth2/token');
        }   
    }
    req.setMethod('POST');
    req.setBody('grant_type=password'+
        '&client_id='+' CLIENT_ID'+
        '&client_secret='+'CLIENT_SECRET'+
        '&username='+customer.thisOrg.username__c+
        '&password='+customer.thisOrg.password__c
    );

    httpResponse res = new httpResponse();

    try {
        // send request     
        Http http = new Http();
        // capture response
        res = http.send(req);
        system.debug('authentication response: '+ res.getBody());
    }
    catch (exception e) {
        system.debug('error authenticating with customer: '+ e.getMessage());
        customer.log += '\n\n error authenticating: ' + e.getMessage(); 
          return null;
    }  

Answer 1

It appears that the Remote Site is case sensitive (not sure why) so if you encounter this error please verify the casing.