nupgrade 3 fails on converting mails from outlook-pst-files if there are !! in the subject of the mails. the mails are converted correctly until the first mail containing !! is reached. here the migration stops. after deleting the converted mails in outlook you can repeat the process ever and ever again.
if there are a lot of such mails it will take a lot of time but the process can be finished.
i have not tested it with !! in the filename of attachments but the process should stop there, too.
the reason lies in the notes-interpretation of !! as separator between servername and filename.