Part of the process of upgrading to phpBB 3.0 is to migrate the password database.
All passwords are held in a one-way encrypted form, so there is no way that anyone can see the "plain text" version. Unfortunately during migration, certain character-sets can "break" the encrypted password, so when you attempt to login you will get an error message.
The easiest way to fix this is to click on the "I forgot my password" option, to which you enter your username and e-mail address. A new (working) password will be sent to you along with a link to activate it. Then you should be able to login and then reset you password to your own choice.
Sorry about this, it is how phpBB works - if it was up to me i would only allow monochrome 7-bit ASCII but i am a bit of a Luddite

Cheers,
Martin