If your users have recently been experiencing login failures with your QBO integrated app here's some guidance from William in Developer Relations to help you resolve this:
If you are seeing QBO logins failing, please make sure you are using the correct login and data URL's.
If you send in a <QBXML> type request to https://merchantaccount.quickbooks.com/j/AppGateway it will fail.
Please make sure you are using the correct URL's for QBO – https://apps.quickbooks.com/j/AppGateway or https://webapps.quickbooks.com/j/AppGateway (if it’s a hosted app).
The https://merchantaccount.quickbooks.com/j/AppGateway and https://webmerchantaccount.quickbooks.com/j/AppGateway will only work for <QBMSXML> type of requests which are meant for developers building apps for QBMS SDK.
Also for connection ticket generation QBMS SDK developers should use the SDK Connection UI hosted by merchant account at (https://merchantaccount.quickbooks.comj/sdkconnection) while the QB SDK developers should use the QBN Login URL hosted https://login.quickbooks.com/j/qbn/sdkapp/confirm?appid=<myAppid>&serviceid=2004&appdata=<myAppData>.