You receive this error message when you attempt to create new folders in your IMAP mailbox:
"Cannot create the folder. The server responded: Serious error while processing CREATE (MsLimitNumFolders (74/76))"
As the error message reads, you have hit the server limit for creating new folders.
This is a limitation of some IMAP servers. Most of the users who report this error are using Charter.net email accounts but other email providers may have the same limitation. Unfortunately, at this time I'm not aware of any good solutions for the problem as it is how your IMAP server is configured. If you need to create new folders, review your existing folders and see if any can be arrived locally or merged with other folders so you can reduce the number of folders in your mailbox.