When running migration over a mailbox, all internally received Notes mails are reverted to plain text.
Externally received Internet HTML mails are, however, properly migrated and converted.
CMTProxy console raises an error "Mime to CD error..." for each processed Lotus Notes mail.
btmail.box in Notes Data directory is either corrupted or locked.
Removing the file. Notes Migrator will create the file again and the problem should be resolved.