Error
Folder Contains Excess Whitespace
Cause
Certain IMAP systems (imap.googlemail.com, etc.) do not allow folders to contain duplicate whitespace characters.
For example, trying to create a folder called "A B", with two spaces between "A" and "B", may result in this error.
Resolution
Rename or filter source mailbox folders to avoid duplicate whitespace characters, and resubmit your migration.