Please try to make a 0.1 mbtc bet and try to partially cashout it, you will know what I am saying.
I understood your issue at the very beginning. What I am saying is that they have different front end for different territory (ex: for .co.uk you will see a very different interface) and the front-end your have for you may get have some coding big or something which is causing the issue.
Anyway, did you contact in their official thread. If not yet then you can post this in there too. Sportsbet has good rep to resolve things very quickly.