Authentication failed - only one address

Hi there, suddenly Em client pops up the message "Password required - server says “[AUTHENTICATIONFAILED]”. I tried everything, changing the passwords, ports, various combinations and it still doesn’t work. It is very strange because only one email address has these problems, other addresses on the same domain are working just fine.

To be even more strange, every now and then the client starts to download messages, and then it stops again.

Please help!