I'm using TSM with EMC Data Domain VTL option.
I've encountered an error during NDMP backup -

ANR8985E The drive VDRIVE16 (/dev/rmt25) in library DD_VTL01 is using an ecryption method that is incompatible with the current server settings. (SESSION: 76587, PROCESS: 367)

ANR8355E I/O error reading label for volume B04510 in drive VDRIVE 16 (/dev/rmt25). (SESSION: 76587, PROCESS: 367)

ANR8779W Scratch volume b04510 changed to Private Status to prevent re-access. (SESSION: 76587, PROCESS: 367)

ANR8507W Volume is using an ecryption key that is not unique and therfore is not secure. (SESSION: 76587, PROCESS: 367)

ANR8302E I/O error on drive VDRIVE16 (/dev/rmt25) with volume B04510 (OP=SETKEY, Error Number=22, CC=0, rc = 1, KEY=05, ASC=24, ASCQ=00, SENSE= .C0.00.03, Description=An undetermined error has occured). Refer to Appendic C in the 'Messages' manual for recommend action. (SESSION: 76587, PROCESS: 367)

This error occured 3 times. Each time causing all of the scratch volumes remaining in the library to change into 'Private' - eventually leaving the virtual library (Containing approximately 4000 scratches) with no scratches at all.

My DRIVEENCRYPTION option in the device class(LTO1) was set to ALLOW. After this happened at the first time, i read about the DRIVEENNCRYPTION options and realized that DRIVEENCRYPT shouldnt be ALLOW if my lto class isn't LTO4 or LTO5. I changed it to 'OFF' and updated all of the relevant volumes into SCRATCH again. Then I waited to see if the problem pops up again.
It did, Twice. But! The problem isn't consistent. It took a lot of successfull backups(and a couple of days) until the error came up once again - Each time while using different VDRIVE and while trying to backup different NAS volume.

What could be the problem?