You removed an UNSRoot item in Identity Manager which was not in use anymore. After that Quick connect synchronization fails although the deleted item is not related in any way to that synchronization. You receive a message like:
Synchronization steps aborted. Details: Cannot access this container: the container ID ‘<Key><T>UNSRoot</T>………..’
Use the Identity Manager tool “Object browser”. Select your UNSRoot item and then select from the main menu: Object | Delete with dependant objects.