Hi,
the problem might be caused by a SCSI problem with the devices/changer. You need to check your fibre channel settings for the VTL used and presented to the host. Check if zoning is correct and server is connected. Use LTT Tools to try connecting to the VTL. Use "uma -ioctl <SCSI address of changer> -barcode" to see if you can connect to the VTL. Use "devbra -dev" to check if the devices are displayed as expected. In DP GUI, check if SCSI address for the changer and devices have changed.
Best regards
Daniel