I've seen the same error on some of my scripts after migrating from Apache 1.* to Apache 2, and as those mentioning it before me, have ruled out any other reasons for failure. It seems indeed something to do with Apache 2's new filter system. So far the only solution I've come up with is rewriting the scripts (I wasn't very proud of that code anyway...). If someone finds another solution I'd be more than happy to hear it.