Diagnosing issues with the system(command) call?

I have an application that at some point calls

system("/usr/sbin/iptables -t nat F")

to flush the ip tables. Since the application is multi threaded and developed by multiple developers, at some point the iptables stopped being set. It turned out that I'm not able to lunch external processes (have also tried system("/bin/ls") without success). The commands can be run from the shell with no issues.

I get the return value -1 regardless of the command called. Is there a way to get some more information on why this is failing?