IMAP session unexpectedly finishes

Question

I work with yahoo imap server. Everything works fine for some time, then several LIST queries return with error imapclient got NO in response: Server returns NO for "A00393 LIST "" "*"": LIST Invalid credentials, then "the last" LIST query return with error imapclient general error: Fatal server ERROR after "A00394 LIST "" "*"": IMAP4rev1 Server logging out. Then new imap session starts and everything works fine.

1. How can I get Invalid credentials error and in new session everything works fine with the same credentials?
2. Can this be problem from server side? How to validate this?