SPException "User cannot be found" when executing SharePoint initial synchronization:
 Error executing synchronization.
 Error executing synchronization project (SharePoint farm (SharePoint_Config_2015))'s workflow (Initial Synchronization).
 Error executing projection step (Group) of projection configuration (Initial Synchronization (Initial Synchronization)).
 Error executing synchronization step (Group)!
[System.AggregateException] One or more errors occurred.
 The mapping rule (vrtMember_Users) was unable to execute synchronization between system objects (xxx) and (groupname=xxx;site=http://xxx.xxx.xxx;type=SPGroup) successfully!
 Error resolving references of property (Users@SPGroup) for system object (groupname=xxx;site=http://xxx.xxx.xxx;type=SPGroup)!
[System.ServiceModel.FaultException] Exception of type Exception occured. Message:
Error reading property Author of https://xxx.xxx.xxx/xxx
Inner exception is of type SPException. Message:
User cannot be found."