Although the message said "This operation requires your Active or Owner key or Master password", it never worked, when I used my Active Key.

Now I used my Master Password and it worked! And after this, I tried it again with my Active Key, and it worked again. (@street.yoga: sorry, I abused your address as a test dummy. Hope this was ok for you, please accept the 0.002 SBD as a compensation. haha)

Hope this stays this way!

Thanks for your help, guys!!!