[exim] Log sieve errors

Hi,

I'm writing a Sieve filter script for Exim, but it isn't applied. I
assume there's some error in the script because a previous version of
the script worked fine.

How can I find out what the error in the Sieve script is? I can't find
anything about it in the usual log files.

Exim 4.86_2 #1 on Ubuntu 16.04

-Yves