OK thanks. Now I understand. On my test site, I have no entry for the default language in the array $this->options['domains'], while you have an empty entry.
It means that a disabled input can be filled (or not) in $_POST the two different ways so I have to manage that.
Thanks a lot for all your tests. I will soon release a development version to fix this and will let you know when it will be available. Waiting for this, I guess you can stay with 1.4.3.