Michel,
Can you test the same issue using the Twacker utility from the Twain Working Group? If you don't have Twacker, you can obtain it by using the following link:
http://twain.org/images/docs/Twack_32.msi
You can test it as follows:
1. Run the Twacker utility.
2. From File menu, select "Select Source", and select the Canon 9050C.
3. From Special menu, select Load/Open SM.
4. From Special menu, select Open Source.
5. From Special menu, select Send.
6. In the MSG field, select MSG_SET.
7. In the capability field, select ICAP_ORIENTATION.
8. In the Value field, select the TWOR_PORTRAIT, and press Send, and then press Close.
9. From Special menu, select Close/Unload SM.
10. Repeat the steps from 3 to 7, but instead of MSG_SET, select MSG_GET.
11. Press on Send, and see the returned value.
Do the same test, but instead of TWOR_PORTRAIT, use TWOR_LANDSCAPE.
Does it work without any failure? What is the returned value for each test?