If QuickBooks is not accepting the application security credentials being passed. 

This could happen in following cases 

• The QuickBooks instance is not a paid instance - credentials for trial or developer instances will be rejected.
• The user attempting to grant permission for the connection is not the Master Administrator, Company Administrator, or Accountant user.
• The QuickBooks instance country is not US or Canada.
• The QuickBooks uses cookies to track your logins - if you have multiple QuickBooks accounts this browser/device may be keeping this information an attempting to automatically log you into the account it has a cookie for. If you are not getting a user name/password dialogue when you click the QuickBooks connect button then it is attempting to connect with your QuickBooks instance of record - you could try another browser/device (ipad or desktop).

If none of the cases above apply to you then let us know and we will investigate further.