Well, I don't use malware - never have. It also seems unlikely that FileZilla would change credentials mid-session, even if it does have settings for other sites as well. My best bet would be that there's a bug or a misconfiguration somewhere. I had hoped the server log would have more details about the authentication failure (e.g. did the credentials actually change, or did some other part of the authentication procedure fail?). Anyway, I'm afraid I gave up and removed my account yesterday, so I guess that's the end of the story - at least until the problem reappears for either you or me. If there's malware on my system, I guess I should soon be getting more clues to that effect.