Then, that is your problem. Since the OS does not see the Exchanger control device, then DP is not going to be able to use it
I recommend that you cycle the power on your tape library, which sometimes will fix this problem, and, run the 'ioscsn -fn' again to see if you see the 'autoch' device again. If not, check all your cables to make sure that nothing has worked loose, and everything is terminated. Think about rebooting the Media Agent machine, although that did not go real well last time, but will give it a chance to rediscover the attached devices again, and run the 'ioscan -fn' again.
However, the fact that you can't see the device at the OS-level takes it out of the possibility of this being a Data Protector issue