Log archiving failing. Seems to be wrong timeline

Hello,

We have 2 postgresql servers (v 9.4.2)  master and slave in streaming
replication. The overall cluster is controlled using pacemaker &
corosync and the pgsql cluster agent which handles failover to, and
promotion of, the slave.

Recently a failover occured and I noticed that log archiving was failing
on the master:

cp: cannot stat 'pg_xlog/000000020000000000000002': No such file or
directory
2016-06-30 11:49:48 BST [13816]: [1235-1] db=,user=,client= LOG: archive
command failed with exit code 1
2016-06-30 11:49:48 BST [13816]: [1236-1] db=,user=,client= DETAIL: The
failed archive command was: cp pg_xlog/000000020000000000000002
/mnt/pgsql/data/pg_archive/000000020000000000000002
cp: cannot stat 'pg_xlog/000000020000000000000002': No such file or
directory
2016-06-30 11:49:49 BST [13816]: [1237-1] db=,user=,client= LOG: archive
command failed with exit code 1
2016-06-30 11:49:49 BST [13816]: [1238-1] db=,user=,client= DETAIL: The
failed archive command was: cp pg_xlog/000000020000000000000002
/mnt/pgsql/data/pg_archive/000000020000000000000002
2016-06-30 11:49:49 BST [13816]: [1239-1] db=,user=,client= WARNING:
archiving transaction log file "000000020000000000000002" failed too
many times, will try again later


But the timeline we're on is different:

# /usr/lib/postgresql/9.4/bin/pg_controldata /mnt/pgsql/data
pg_control version number:            942
Catalog version number:               201409291
Database system identifier:           6198394727571912088
Database cluster state:               in production
pg_control last modified:             Thu 30 Jun 2016 11:42:42 BST
Latest checkpoint location:           2/EEE842E8
Prior checkpoint location:            2/EED64F68
Latest checkpoint's REDO location:    2/EEE4B610
Latest checkpoint's REDO WAL file:    0000002C00000002000000EE
Latest checkpoint's TimeLineID:       44
Latest checkpoint's PrevTimeLineID:   44
Latest checkpoint's full_page_writes: on
Latest checkpoint's NextXID:          0/2947680
Latest checkpoint's NextOID:          74375
Latest checkpoint's NextMultiXactId:  464
Latest checkpoint's NextMultiOffset:  929
Latest checkpoint's oldestXID:        677
Latest checkpoint's oldestXID's DB:   1
Latest checkpoint's oldestActiveXID:  2947680
Latest checkpoint's oldestMultiXid:   1
Latest checkpoint's oldestMulti's DB: 1
Time of latest checkpoint:            Thu 30 Jun 2016 11:42:27 BST
Fake LSN counter for unlogged rels:   0/1
Minimum recovery ending location:     0/0
Min recovery ending loc's timeline:   0
Backup start location:                0/0
Backup end location:                  0/0
End-of-backup record required:        no
Current wal_level setting:            hot_standby
Current wal_log_hints setting:        off
Current max_connections setting:      250
Current max_worker_processes setting: 8
Current max_prepared_xacts setting:   10
Current max_locks_per_xact setting:   64
Maximum data alignment:               8
Database block size:                  8192
Blocks per segment of large relation: 131072
WAL block size:                       8192
Bytes per WAL segment:                16777216
Maximum length of identifiers:        64
Maximum columns in an index:          32
Maximum size of a TOAST chunk:        1996
Size of a large-object chunk:         2048
Date/time type storage:               64-bit integers
Float4 argument passing:              by value
Float8 argument passing:              by value
Data page checksum version:           0


Why are we trying to archive logs which belong to an old timeline?

Any thoughts much appreciated.

Regards

Chris