It usually happens when there are bulk records to Sync and the request sending and receiving gets locked, at this time check all the records are synced or not. If records are not Synced reach out to our support by dropping an email to firstname.lastname@example.org or If Sync is successful you can close it by following steps.
Open Task Manager => Start Task Manager => Under the Process Tab => Right Click on the QBWebConnector.exe => Click on the End process tree in the Dialog box.
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