I'm guessing that the IMAP command to "look into a folder" (like the "cd" command in DOS / Windows command line) is somehow failing.
If you renamed some folders, long press the account -> options and folders -> folders to make the app refresh.
However, you hadn't, or if it's the INBOX -- which is supposed to always be there, and is always called INBOX -- then it's most likely a Yahoo error (again).
Feel free to capture the issue in the app's debug log (see link in my signature below).