If you are getting the following error message, it is likely because Port 33333 is being used:
Part of the embedded credentials for QuickBooks Online requires that the CallBack URL match what is configured in QuickBooks Online so that it can be validated. This means that if you are to use the embedded credentials, you cannot alter the CallBack URL or it will always fail.
To resolve the issue, you will need to make sure no other process is running on Port 33333.
If you would like to stop the process that is currently running on Port 33333, you can view all the Ports in current use and the PID of the process using them by running the following command in the Command Prompt program:
netstat -a -o
Once you find the PID of the program currently running on port 33333 of your machine, you can open the Task Manager program and then go to the Services tab:
This will give you a list of the currently running Services on your machine, along with their PID. Find the Service with a PID matching the one you found for the process which is running on Port 33333 and end it, and then reattempt making a connection. Please note you will need to reset the CallBack URL so it is again empty.