P365: Sync error - "Mailbox copy exception Value cannot be null. Parameter name: uriString" (4321161)

We are seeing the following error for a few user object, but not all. What is the cause?

sync error - Mailbox copy exception Value cannot be null. Parameter name: uriString
System.ArgumentNullException: Value cannot be null.
Parameter name: uriString
at System.Uri..ctor(String uriString)
at MapiOHtttplib.MAPIHttpAdatper.SendMAPIHttpRequest(String URL, IRequestBody requestBody, String requestType, CookieCollection cookies, Int32 RequestCount, String RequestId, String ClientInfo, MAPIHttpAdatper MAPIHttpAdatper, MailboxType mailboxType, Control Control)
at MapiOHtttplib.MAPIHttpAdatper.Bind(String UserDn, MailboxType mailboxType)
at MapiOHtttplib.Functions.AddressBookFunctions.GetAddressBookMailboxSettings(String Mailbox, Control Control, MailboxType mailboxType, ExchangeService Service, String ADGuid)
at MapiOHtttplib.Functions.MailboxFunctions.CopyMailbox(String SourceMailbox, String SourceMailboxAdObjectGuid, Credential SourceCredential, String TargetMailbox, String TargetMailboxAdObjectGuid, Credential TargetCredential, Control Control, Boolean Delta, AddressBook TransformAddressBook, ConcurrentDictionary`2 AddressCache, Boolean useInLineAddressBook)

Confirm the user Mailbox is MAPI and EWS enabled.

From our Known Limitations: https://support.quest.com/technical-documents/binary-tree-power365/current/help-center/28#TOPIC-1564865

Mail migration cannot be performed on mailboxes that are assigned an Office365 Kiosk License as MAPI connection is disabled for this license type. As a workaround, assign the mailbox an E1 or higher license during the migration, and then revert to the kiosk license. (8062)

Retry the sync.

Contact Quest Product Support if you have any issues with the above resolution.