These errors
Cannot open exchanger control device (Details unknown.)
where it says 'Details unknown' usually represent a problem with the driver
I would first fun this command on the Media Agent machine 'bkprrb ':
ioscan -fnC Autoch
If nothing comes back, just run
ioscan -fn
and look at the output for 'autoch'. If you find it, make sure that is is Claimed, and that it does not say 'NO_HW'
If it says either, I recommend that you reboot 'bkprrb' so that teh devices can be rediscovered, and, while you are at it, also reboot your tape library
At the command prompt of the server 'bkprrb', run
/opt/omni/lbin/devbra -dev
and see if anythng returns with a category 'EXCH', if it does, confirm the device file as /dev/rchgr/autoch1, see if it has a serial number (S/N), and that it lists the correct drives and number of slots