Watchdog Web Server (Apache) cause apache error

[BigLuck]

I activated some days ago the watchdog module for the Apache Web Server with a 120 seconds control interval. Now looking in the apache error log ( /var/log/apache2/error.log ) I found this error message with regular interval:

[Thu Dec 14 11:28:39 2006] [error] [client #my_server_ip#] Client sent malformed Host header
[Thu Dec 14 11:30:41 2006] [error] [client #my_server_ip#] Client sent malformed Host header
[Thu Dec 14 11:32:44 2006] [error] [client #my_server_ip#] Client sent malformed Host header
[Thu Dec 14 11:34:46 2006] [error] [client #my_server_ip#] Client sent malformed Host header
[Thu Dec 14 11:36:48 2006] [error] [client #my_server_ip#] Client sent malformed Host header

Does anyone know how to solve this annoying problem?

Thanks in advance.