UTF-8 character set

Hi There,

Could it be, that the plugin doesn't work with the UTF-8 character set?
I have a clienent with the mapped domain:
unser-fuhlsbuettel.de and they want to switch to
unser-fuhlsbüttel.de.

But it is not accepted by the plugin. I always end up on my registration page as if the domain doesn't exist.

Any idea?

Thanks
Tee