Login problems with external blogging software

I am trying to get Windows Live Writer working for my blog, but for some reason it cannot log in; it gives me a login/password error. Same for Qumana and w.bloggar. The same login/password combination works fine when I log in through the web interface.

However, if I add a new account, I can log in perfectly well using an external writer — but I would really like to use my existing account.

Any ideas what could cause this behaviour and what I could do about it?

Thanks,
Michael