When Exchange Store Manager attempts to connect to mailboxes that are in Office 365 it is unable to scan them and in the ESM MailboxID-xx-yyyymmdd.wlog you see the following:
00:21:21 DEBUG: Creating AutodiscoverEWS instance by calling: Quest.ArchiveManager.AutodiscoverEWS
00:21:22 ERROR: COMAutodiscoverEWS.Discover2:
00:21:22 [80004005] Unspecified error
00:21:22 Source: Quest.AM.Autodiscover
00:21:22 Description: System.AggregateException: One or more errors occurred.
00:21:22 Inner exception:
00:21:22 System.AggregateException: One or more errors occurred.
00:21:22 Inner exception:
00:21:22 System.NullReferenceException: Object reference not set to an instance of an object.
00:21:22 Source: Quest.AM.Autodiscover
In the ESM log you may also see:
10/3/2018 5:59:51 AM Failed 545 COMAutodiscoverEWS.Discover2: [80004005] Unspecified error Source: Quest.AM.Autodiscover Description: System.AggregateException: One or more errors occurred. Inner exception: System.AggregateException: One or more errors occurred.