You are seeing the below error message on the Sync App or in the log file of the Sync App.
Error System.Net.HttpListenerException (0x80004005): Failed to listen on prefix
Sage 50 users.
QuickBooks Desktop users.
This error is typically caused by having two instances of the service running at the same time.
To resolve this issue you will need to perform the following steps:
Open your PC's Task Manager.
Click on the Details tab.
Search for all instances of the AutoEntry Desktop Sync App.
Right click on each one and select End Task.
Open the Sync App again and the error should clear.