[HELP] Issue with standby server using WAL archive

Hello,

I'm facing this issue with a standby server which is setup to use WAL archive.

Config on Master:
archive_mode = on
archive_command = 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f'

Config on standby:
standby_mode = 'on'
restore_command = 'cp /mnt/server/archivedir/%f %p'

Before starting standby, I took base backup

pg_basebackup -v -D main -R -P -h pg1 -p 5432 -U replication
pg_basebackup: initiating base backup, waiting for checkpoint to complete
pg_basebackup: checkpoint completed
43518/43518 kB (100%), 1/1 tablespace                                        
NOTICE:  pg_stop_backup complete, all required WAL segments have been archived
pg_basebackup: base backup completed

Now, copied recovery.conf to data directory

Started postgres server on standby and seeing below error in log.

2020-07-02 13:14:02.134 UTC [7643] postgres@template1 FATAL:  the database system is starting up
2020-07-02 13:14:02.644 UTC [7646] postgres@template1 FATAL:  the database system is starting up
2020-07-02 13:14:03.154 UTC [7649] postgres@template1 FATAL:  the database system is starting up
2020-07-02 13:14:03.664 UTC [7652] postgres@template1 FATAL:  the database system is starting up
2020-07-02 13:14:04.174 UTC [7655] postgres@template1 FATAL:  the database system is starting up
cp: cannot stat '/mnt/server/archivedir/00000001000000000000000D': No such file or directory
2020-07-02 13:14:04.684 UTC [7660] postgres@template1 FATAL:  the database system is starting up
2020-07-02 13:14:05.194 UTC [7663] postgres@template1 FATAL:  the database system is starting up
2020-07-02 13:14:05.197 UTC [7664] [unknown]@[unknown] LOG:  incomplete startup packet
cp: cannot stat '/mnt/server/archivedir/00000001000000000000000D': No such file or directory
cp: cannot stat '/mnt/server/archivedir/00000001000000000000000D': No such file or directory

On Master:
postgres@pg1:~$ ls -lrt /var/lib/postgresql/9.6/main/pg_xlog/
total 65544
-rw------- 1 postgres postgres 16777216 Jul  2 13:09 00000001000000000000000E
-rw------- 1 postgres postgres 16777216 Jul  2 13:13 00000001000000000000000F
-rw------- 1 postgres postgres 16777216 Jul  2 13:13 00000001000000000000000C
-rw------- 1 postgres postgres      302 Jul  2 13:13 00000001000000000000000C.00000028.backup
drwx------ 2 postgres postgres     4096 Jul  2 13:18 archive_status
-rw------- 1 postgres postgres 16777216 Jul  2 13:18 00000001000000000000000D

postgres@pg1:~$ ls -lrt /var/lib/postgresql/9.6/main/pg_xlog/archive_status/
total 0
-rw------- 1 postgres postgres 0 Jul  2 13:13 00000001000000000000000C.done
-rw------- 1 postgres postgres 0 Jul  2 13:13 00000001000000000000000C.00000028.backup.done

On standby:
postgres@pg3:~$ ls -lrt /var/lib/postgresql/9.6/main/pg_xlog/
total 16388
-rw------- 1 postgres postgres 16777216 Jul  2 13:13 00000001000000000000000C
drwx------ 2 postgres postgres     4096 Jul  2 13:13 archive_status
postgres@pg3:~$ ls -lrt /var/lib/postgresql/9.6/main/pg_xlog/archive_status/
total 0
-rw------- 1 postgres postgres 0 Jul  2 13:13 00000001000000000000000C.done

/mnt/server/archivedir/ is a NAS mount and is accessible from master and standby.

postgres@pg3:~$ ls -lrt /mnt/server/archivedir/
total 32788
drwx------ 2 postgres postgres    16384 Jul  1 14:44 lost+found
-rw------- 1 postgres postgres 16777216 Jul  2 13:13 00000001000000000000000B
-rw------- 1 postgres postgres 16777216 Jul  2 13:13 00000001000000000000000C
-rw------- 1 postgres postgres      302 Jul  2 13:13 00000001000000000000000C.00000028.backup