"internal error" on ticket creation / empty owner?

Dear List,

We have just set-up RT 3.0.10 for win32. Now if we want to add a new bug
we get an “internal error” on ticket creation if the owner drop-down box
is not changed. This drop-down box contains one empty field which IMHO
is pre-selected and causes this error.
How can we get rid of this empty field in the owner box?

Thanks and happy easter!