Received legacy SPC-2 RESERVE/RELEASE while active SPC-3 registrations exist, returning RESERVATION_CONFLICT

I have 2 iSCSI fileio backstores.

I try to Determine whether shared disks can be used as tiebreaker disks . For that I run on the iSCSI initiator a tool from IBM to release the reservation:

[root@softiwarp-1 ~]# /media/distr-ro/db2/10.5/linuxx64/v10.5fp7_linuxx64_universal_fixpack/universal/./db2/linuxamd64/install/tb_break -u -t SCSI WWID=`scsi_id -g -u /dev/sdd`
Initializing SCSI tie-breaker (WWID=36001405bb9264c9e8c145429dbc76047)
Releasing tie-breaker (WWID=36001405bb9264c9e8c145429dbc76047)
tb_release status 0 (errno=0)
[root@softiwarp-1 ~]# /media/distr-ro/db2/10.5/linuxx64/v10.5fp7_linuxx64_universal_fixpack/universal/./db2/linuxamd64/install/tb_break -u -t SCSI WWID=`scsi_id -g -u /dev/sde`
Initializing SCSI tie-breaker (WWID=36001405ed3a0a03628a4108bc3e24a82)
Releasing tie-breaker (WWID=36001405ed3a0a03628a4108bc3e24a82)
tb_release status -1 (errno=25)

For one of the disks the command succeeds. The other fails and on the target in dmesg I see

Received legacy SPC-2 RESERVE/RELEASE while active SPC-3 registrations exist, returning RESERVATION_CONFLICT

What does it mean?

/iscsi/iqn.20...ample:t1/tpg1> ls /
o- / ......................................................................................................................... [...]
  o- backstores .............................................................................................................. [...]
  | o- block .................................................................................................. [Storage Objects: 0]
  | o- fileio ................................................................................................. [Storage Objects: 2]
  | | o- shareddata ................................... [/home/libvirt/images/default/test_iSCSI.img (20.0GiB) write-back activated]
  | | o- shareddata_tb .............................. [/home/libvirt/images/default/test_iSCSI2.img (100.0MiB) write-back activated]
  | o- pscsi .................................................................................................. [Storage Objects: 0]
  | o- ramdisk ................................................................................................ [Storage Objects: 0]
  o- iscsi ............................................................................................................ [Targets: 1]
  | o- iqn.2014-08.com.example:t1 ........................................................................................ [TPGs: 1]
  |   o- tpg1 ............................................................................................... [no-gen-acls, no-auth]
  |     o- acls .......................................................................................................... [ACLs: 1]
  |     | o- iqn.2014-08.com.example:client ....................................................................... [Mapped LUNs: 2]
  |     |   o- mapped_lun0 ........................................................................... [lun0 fileio/shareddata (rw)]
  |     |   o- mapped_lun1 ........................................................................ [lun1 fileio/shareddata_tb (rw)]
  |     o- luns .......................................................................................................... [LUNs: 2]
  |     | o- lun0 ................................................ [fileio/shareddata (/home/libvirt/images/default/test_iSCSI.img)]
  |     | o- lun1 ............................................ [fileio/shareddata_tb (/home/libvirt/images/default/test_iSCSI2.img)]
  |     o- portals .................................................................................................... [Portals: 1]
  |       o- [::0]:3260 ....................................................................................................... [OK]
  o- loopback ......................................................................................................... [Targets: 0]