If you don't have any custom logic for panic log (i.e. /usr/local/lib/exim4/nonzero_paniclog_hook), you can work around this issue by editing /etc/cron.daily/exim4-base and changing line 18 from:
E4BCD_PANICLOG_NOISE=""
to
E4BCD_PANICLOG_NOISE="DKIM: Error while running this message through validation, disabling signature verification.$"
If you don't have any custom logic for panic log (i.e. /usr/local/ lib/exim4/ nonzero_ paniclog_ hook), you can work around this issue by editing /etc/cron. daily/exim4- base and changing line 18 from:
E4BCD_ PANICLOG_ NOISE=" "
to
E4BCD_ PANICLOG_ NOISE=" DKIM: Error while running this message through validation, disabling signature verification.$"
--
Steve