From DP end:
[Normal] From: BSM@cellmanager "archivelog" Time: 01/30/2020 10:18:04 AM
Backup session xxx started.
[Normal] From: BSM@cellmanager "archivelog" Time: 01/30/2020 10:18:05 AM
OB2BAR application on "server1_dns1" successfully started.
[Normal] From: BSM@cellmanager "archivelog" Time: 01/30/2020 10:18:08 AM
OB2BAR application on "server1_dns1" disconnected.
[Critical] From: BSM@cellmanager "archivelog" Time: 01/30/2020 10:18:08 AM
None of the Disk Agents completed successfully.
Session has failed.
From DB2 log:
FUNCTION: DB2 UDB, data protection services, sqlpgArchiveLogFile, probe:3160
MESSAGE : ZRC=0x86100025=-2045771739=SQLP_MEDIA_VENDOR_DEV_ERR
"A vendor device reported a media error."
DATA #1 : <preformatted>
Failed to archive log file Sxxx.LOG to VENDOR chain 0 from /db2/tws_log/NODE0000/LOGSTREAM0000/.
On DB2 debug: it looks like the db2 api tried to get the config file with its other name:
[ 99] 2020-02-18 04:30:13.703 ("/integ/barutil/ob2util.c $Rev$ $Date:: $:":550)
[ 99] <<=== (2) } /* BU_GetConfig */
[ 99] RetVal: (int) 8
[ -2] BU_GetConfig(cellmanager, server1_dns2, DB2, db2inst1, config) failed with error  Object not found or access denied.
As the Cell Manager only has its server1_dns1 name, backup was unable to proceed.
running omnicheck -dns -host server1_dns1 always returns:
CM connects to server1_dns1 but connected system presents itself as server1_dns2
Config the backup as server1_dns2, if not possible, create 2 different config files as server1_dns2 name