Fetchmail doesn't get killed after being run by cron

[Robert-Jan Kuijvenhoven <r.kuijvenhoven@kabelfoon.nl>]

Hello,

I am running fetchmail as a cron job every half hour. It worked fine until 
a few days ago. Suddenly, I started to receive the following messages:

> Subject: Cron <root@linbox> /usr/bin/fetchmail -f /etc/fetchmailrc 
> --syslog --invisible
> X-Cron-Env: <SHELL=/bin/sh>
> X-Cron-Env: <MAILTO=frank>
> X-Cron-Env: <HOME=/root>
> X-Cron-Env: <PATH=/usr/bin:/bin>
> X-Cron-Env: <LOGNAME=root>
> Date: Sun, 26 Nov 2000 22:00:01 +0100
>
> fetchmail: another foreground fetchmail is running at 853.

I have to kill fetchmail by hand to get fetchmail to receive messages again.

Fetchmail checks five mailboxes from three different isp's. I have checked 
/var/log/syslog and I noticed that fetchmail stalls after checking the 
first mailbox (or after starting to check the second one). There is no 
error message in /var/log/syslog. There was no mail in the first mailbox 
and there (probably) was no mail in the second mailbox as well.

What could be the problem here?

TIA,

Robert-Jan Kuijvenhoven