Re: wrong password
I think the reason it was correct sometimes would be due to the content of the password. bbPress originally (or a while ago anyway, pre-0.9*) used a 6 character alpha-numeric password. Now, it uses something more complex, and depending on what characters are in the password, maybe some certain characters get dropped in that plugin. If you post a link to the plugin you’re using, I can look at it and see if this is likely. You could also contact the plugin author and see if they have any idea.
On the special characters in the password email. I suspect the message is just sent as plain text, but those are html entities. I didn’t look too hard at it. Maybe you could just put the è in the text of the message and hope the email client can handle it.