Replication terminated by primary server

Hi,
we have configured a synchronous master-slave replication.
Node "postgres1" is the master, while node "postgres2" is the slave.

It works fine, but when restarting the master (using "pg_ctl restart"),
the replication is broken. In the slave log we have the following message:

2012-07-16 13:10:20 CESTFATAL:  replication terminated by primary server

There is a process in the slave which indicates the file the slave is
waiting for:

[postgres@postgres2 walpostgres1]$ ps -ef | grep start
postgres  1874  1872  0 Jun19 ?        00:01:04 postgres: startup
process   waiting for 000000010000000C000000A6

Once we manually copy this file from the master (in path
$PGDATA/pg_xlog) to the archive location (in the slave), the replication
is resumed.

[postgres@postgres1 pg_xlog]$ scp 000000010000000C000000A6
postgres2:/home/postgres/walpostgres1
000000010000000C000000A6 100%   16MB  16.0MB/s   00:01

And in the slave log:
2012-07-16 13:16:03 CESTLOG:  restored log file
"000000010000000C000000A6" from archive
2012-07-16 13:16:03 CESTLOG:  record with zero length at C/A60000B0
2012-07-16 13:16:03 CESTLOG:  unexpected pageaddr C/84000000 in log file
12, segment 166, offset 0
2012-07-16 13:16:03 CESTLOG:  streaming replication successfully
connected to primary

Is this a configuration issue?

The archive_command in postgresql.conf in the master is:
archive_command = 'scp %p postgres2:/home/postgres/walpostgres1/%f'

Thank you
Best regards,
Nicolau