[Exim] exim crash on filter

I wrote up the following filter to register and audit emails between one group at a company and another because they are not allowed to share certain kinds of information for legal reasons. Most fo the time it works without a hitch - the problem is after the upgrade to 4.20 it crashes/hangs exim with a fatal error - particularly when delivering a message to a large number of people. I have tried rewriting it using different constructs (if " ... addresses ..." contains $sender_address. etc) with no luck. Does anyone have any idea what might be causing exim to explode on such a filter?

# Send a warning mail if mail is sent between corpfin and research
if (foranyaddress " ... corpfin addresses here, cut for privacy ... " ("$thisaddress" matches "$sender_address")) and (foranyaddress " ... research addresses here, cut for privacy ... " ("$thisaddress" matches #"$local_part@$domain")) then
        mail to "audit address"
        from "admin@???"
        subject "Chinese Wall Alert"
        text "Subject: $h_subject:\nFrom: $h_from:\nTo: #$local_part@$domain:\n\n$message_body"
endif

-David Powers