Hi.
Depending on your RT version you might have hit a bug with line break parsing: Set Owner in Create Ticket using API - #10 by Andrew_Huddleston
There are other hints within this thread:
Some hints:
is andrew a Name of an RT user
is andrew account enabled
as andrew OwnTicket right on queue “desk”
what if you user Owner instead of owner
what if you use unix line breaks (%0A) instead of %0D%0A
Bye, Daniel