BRMS fails again mysteriously

Another question about the curse of my existence, BRMS.

The job starts and fails:

12/02/17 21:18:15 BRM1411 10 *SAVSECDTA and *SAVCFG issued for control
group BRMSMTH. Q1ACSS BRMSOFR *BKU
12/02/17 21:18:15 BRM4201 10 Change to media controls
successful. Q1ACST BRMSOFR *BKU
12/02/17 21:18:15 BRM4201 10 Change to media controls
successful. Q1ACST BRMSOFR *BKU
12/02/17 21:18:16 BRM1457 10 Starting SAVSECDTA to devices
TAPMLB01. Q1ACCFG BRMSOFR *BKU
12/02/17 21:18:20 CPF3794 70 Save or restore operation ended
unsuccessfully. Q1ACCFG BRMSOFR *BKU
12/02/17 21:18:22 BRM1820 99 Control group BRMSMTH type *BKU ended
abnormally. Q1AC0BK BRMSOFR *BKU

There is an error in QSYSOPR log:

Message . . . . : No media of class ULTRIUM3 is available at
location
TAPMLB01. (C
G)
Cause . . . . . : No media of class ULTRIUM3 exists or is available for
the
operation to use at location TAPMLB01.

There are lots of Ultrium 3 tapes in the tape library in *EXP state, e.g,:

EVR480 *EXP 10/21/17 11/25/17 TAPMLB01 11/22/17 ULTRIUM3
EVR486 *EXP 11/15/17 11/23/17 TAPMLB01 11/14/17 ULTRIUM3

etc.

The device is varied on, yes I checked.

What does this piece of unspeakable dross want from me this time?