Mailbox does not exist:
Some defective IMAP servers may list folders but later fail to return content from these folders. This typically affects IMAP implementations that are unable to handle special characters in folder names. For example, a defective IMAP server may list a folder called "A - B", but fail to return content for this folder because of the special character ("-" in this example). The problem may also affect folders having trailing spaces. For example, a defective IMAP server may list a folder called "A/B ", but fail to return content for this folder because of the trailing space.
This error will result in a failed migration because an entire folder cannot be processed. Consider renaming mailbox folders to avoid special characters. Consider specifying a folder exclusion filter to exclude these folders from the migration. Consider upgrading the IMAP system to see if a more recent version resolves the problem.