If you ever encounter this error, when migrating an M365 mailbox to an onprem DB, it is due to the fact that the AD attribute “mDBUseDefaults” isn’t set to False.
If the Attribute editor for the specific user doesn’t display the attribute “mDBUseDefaults” there is a filter option “Show only attributes that have values”. Click into “filter” and unselect it, then find “mDBUseDefaults” and change its value to false, then try and migrate the user.
