Well the behavior was different in that particular case. Upon form submission, you’d end up with either a blank page or be directed to the site’s homepage. In your case (if it’s the same as the post that started this thread), you’re being returned to the form you’re trying to submit. I doubt the trouble with yours is related to a mod_security issue or you’d be seeing the same effects.
In your case, you said you’re seeing the problem intermittently. Does that mean that the same form will sometimes do this and other times will submit successfully? Or is there a consistent pattern you’re able to discern where some forms will always submit, others will always return to the form without properly submitting?