Agfa Duoscan T1200 error msg

I bought an Agfa Duoscan T1200 scanner along with a SCSI-USB converter
to scan some antique glass plate negatives. It works find in
reflective mode, but in negative or transparancy mode, I get the error
message: "A calibration error has occurred. Make sure the calibration
hole of the transparency tray is not covered and that the transparency
tray is fully inserted." This error message comes up both when the
tray is fully inserted and when it is fully removed. I assume the slot
in the front of the transparancy tray is the calibration hole. It is
not blocked.