A migration fails because Notes Migrator cannot create a folder because of source data corruption. The error in the log will be:
! 13:23:56 Error: MAPI Create Folder --Corrupt data 8004011b
! 13:23:56 Error: Critical Failure while creating folder:
The source mail folder is corrupt. If the admin pulls the folder up in Domino Designer and edits the folder, you will notice the folder does no have a name. In edit mode the folder will appear at the top of the list and will display as "Untitled".
Right click the Designer pane while in edit mode , select "folder properties, the Name field is blank in the dialog box. Filling in the name manually and save the folder. This will resolve the error.
How to find the corrupt folders
Open the 'Normal' migration log, the problem folders will have a number of carriage returns, creating spaces between each folder row and the next. You will also notice there is no document count. These are the folders to remediate.