How to read mainframe file located on a tape?

[PeterHH]

Dear all,

we have currently encountered a problem when reading mainframe file stored on a tape. The error message doesn't say too much to us:
"Error occurred when opening file //'LV.PROD.LHPBC43.LAOQB36.INV06' in mode "rb,type=record"; Error: "EDC5061I An error occurred when attempting to define a file to the system."

Our environment:
We use DataStage on USS version 7.5.1A. Reading is being accomplished using a CFF stage. We tested successfully the access to the same file located on the disk. We also tested access rights and this is not the cause of the problem since we can access the file on the tape under authority of the DS user per FTP. Resumed:
1. Record structure seems to be OK (tested on the same file on the disk)
2. Access rights are OK (tested per FTP)
3. We see in the system log that the file was mounted on Unix

Has perhaps anyone an idea what goes here wrong?
Your help would be greatly appreciated.

Kind regards
Peter Mandzak