Internal Server Error when clicking Acknowledge Alert only for one group of hosts

(xymon)@xymon:~$ tail -1 /var/log/apache2/error.log
[Wed Apr 05 14:15:17.305430 2017] [cgi:error] [pid 3744] [client
10.13.136.222:40560] End of script output before headers: acknowledge.sh,
referer:
https://xymon.example.net/xymon/groupA/groupA_syslog/groupA_syslog.html


page groupA GroupA SERVERS
subpage groupA_syslog  GroupA Syslog Servers
group-compress <H3><I>GorupA Syslog SERVERS</I></H3>
192.168.1.250 foo.example.net
192.168.2.250 bar.example.net

If I am a layer up Acknowledge Alert works without giving any internal
server error.

Any suggestion on where to look?

Thanks

-- 
Asif Iqbal
PGP Key: 0xE62693C5 KeyServer: pgp.mit.edu
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?