Error backing up cluster, The specific server index is invalid

[26/Apr/2016 05:16:26] INFO - Task bucket_compaction successfully completed

[172.23.96.123] Executing task 'cbbackup'

[26/Apr/2016 05:16:26] INFO - cbbackup into /data/cbbackup_dir

[172.23.96.123] run: rm -rf /data/cbbackup_dir

[172.23.96.123] run: /opt/couchbase/bin/cbbackupmgr config --archive /data/cbbackup_dir --repo default

[172.23.96.123] out: Backup repository `default` created successfully in archive `/data/cbbackup_dir`

[172.23.96.123] out: 

[26/Apr/2016 05:16:26] INFO - /opt/couchbase/bin/cbbackupmgr backup --archive /data/cbbackup_dir --repo default --host http://172.23.96.117:8091 --username Administrator --password password --threads 16

[172.23.96.123] run: /opt/couchbase/bin/cbbackupmgr backup --archive /data/cbbackup_dir --repo default --host http://172.23.96.117:8091 --username Administrator --password password --threads 16

[172.23.96.123] out: 

[172.23.96.123] out: 

[172.23.96.123] out: 

[172.23.96.123] out: 

[172.23.96.123] out: 

[172.23.96.123] out: Backing up to 2016-04-26T05_16_27.049344256-07_00

[172.23.96.123] out: Copying at 0B (estimating time remaining)                           0 items / 0B

[172.23.96.123] out: bucket-1                [                                              ] 100.00%

[172.23.96.123] out: Error backing up cluster: The specific server index is invalid.

[172.23.96.123] out: 

Fatal error: run() received nonzero return code 1 while executing!