don't know if this is the right place for bug-reports, but
on German language systems, the @ sign is typed using a
special key (AltGr) which usually functions as a combination of Alt
and Ctrl. So @ is a combination of Alt + Ctrl and Q.
Kuler desktop seems to omit the Alt key in that sequence and
quits if you want to enter your E-Mail adress in the login dialog.
Copy/Paste is a way to work around it but nevertheless I think this
is a bug.