[check_postgres] Error message without explaination

[Gianpaolo Pedrazza]

Hi,
I would like to monitoring some psql-instances (9.1.1, server 8.4.8)
running on Debian 6.0. 
My configuration on the monitoring server is:
- Ubuntu 11.10 64 bit
- Groundwork Monitor Community Edition 6.0.1 64 bit
- check_postgres-2.18.0
- postgresql-client-9.1   

My command is:
/usr/local/groundwork/nagios/libexec/check_postgres -H 10.1.102.106
--action connection --dbuser postgres --dbpass XXXXXX 

When I run from the shell, it works as expected:
POSTGRES_CONNECTION OK: DB "postgres" (host:10.1.102.106) version 8.4.8
| time=0.60s

But when I try it from Groundwork, I get only: 
ERROR: 
Command returned exit status 3

When an older version of psql server (such as 8.3) is running on the
client side, it works fine.
I tried to replace postgres-client (9.1) with an older version (8.4),
but it was unsuccessful. Despite I'm not a Perl-expert, I tried to find
where is the problem on the script. On line 2274

eval {
	alarm $timeout;
        $res = system $PSQL => @args;
};

Although the arguments are exactly the sames, running the script from
the bash I get res==0, from groundwork res==11. Can someone suggest what
i could do to understand what it's getting wrong? 

Thanks,
Gianpaolo