[Exim] Exim Autoresponder started failing.

I have read through the manual/FAQ's regarding this question before posting.

I am running an autoresponder, as follows, for incoming email tagged
'personal':

<begin>
# Exim Filter
if personal then
mail
to $reply_address
subject "Re: $h_subject"
text "Thank you for emailing mail@??? \n\n etc etc
log
once
once_repeat 7d
endif
<end>

Until recently, everything's worked fine. Incoming email addresses have
been appended to a logfile called 'once' and the sender has only received
one email autoresponder every seven days.

It stopped working all of a sudden. No updates to exim have been performed
and all the ownership permissions are correct. Exim now acts as if the
'once_repeat 7d' wasn't there. Every email coming in is being
autoresponded to.

Never seen this type of behaviour before.

Can anybody help assess this issue / tell me what the problem might be,
please?

Many thanks