MOD_SEC or ModSecurity is a web application firewall that can work either embedded or as a reverse proxy. It provides protection from a range of attacks against web applications and allows for HTTP traffic monitoring, logging and real-time analysis.
It can be set to operate varying levels of security. In some very rare situations it can interrupt program flow. It is hard to diagnose these issues but when you are sure it's the problem you have two options:-
1. Disable or modify features in the script to comply with the rules set by your host.
2. Ask you host to allow an exception. Most hosts will do this without any problem.
In nearly all circumstances it is possible to fix this issue with or without your hosts intervention and still provide 100% service from your script. Unfortunately as mentioned above it is difficult to know whether the problem you are experiencing is due to MOD_SEC, the best advice that it is probably not however to help you diagnose the issue the following forum post provides the symptoms and a solution. Digesting this may help:-
http://forum.ventrino.com/index.php?showtopic=7651&st=0&p=39837&#entry39837