Urgent! Log full with inexistent PID processes

Hi everybody.

We have a postgres 8.4.5-2 database under Debian Lenny 64bit.
This database has been working without any problem, but this afternoon
server has got out of space due to a log dump.

Log file gets full with this entry until it reaches several gigabytes:

2011-06-27 13:25:07 CEST LOG: PID 21822 in cancel request did not match
any process
2011-06-27 13:25:07 CEST LOG: PID 21821 in cancel request did not match
any process
2011-06-27 13:25:07 CEST LOG: PID 21820 in cancel request did not match
any process
2011-06-27 13:25:07 CEST LOG: PID 21816 in cancel request did not match
any process
2011-06-27 13:25:07 CEST LOG: PID 21802 in cancel request did not match
any process

Of course, we have restarted postmaster and rebooted machine but problem
remains.

Any clue about this?

pg_hba only allows connection from localhost and, of course, there are
no processes with these PIDs in the server...

Thanks in advance.