Sometimes your IMAP4 email client may show folders in the folder tree, but when you select them, you get an error that the folder does not exist.
This happens because IMAP4 has the concept of being able to 'subscribe' to folders - this means that you can hide folders by unsubscribing from them.
However, for various reasons, IMAP4 says that if a folder disappears it must not be automatically unsubscribed. This means that if a folder was deleted it may still be in the subscribe list. Some email clients may then still show that folder, even though it no longer really exists.
You can solve this problem by manually unsubscribing from the folder. How you do this depends on the email client. In Mozilla Thunderbird, for example, you right-click the account name in the folder tree. Then click on 'Subscribe'. You are then shown a folder list, with checkboxes showing the folders you are subscribed to - you can uncheck the non-existent folders.