[exim] Sieve filter not working properly anymore, how to debug?

[Yves Goergen via Exim-users]

Hello,

I'm setting up a new mail server with Exim and try to get the Sieve 
filter working as it does on another server. But it doesn't. And I don't 
know why.

Here's a sieve script for demo@test.local:

if header :contains ["Subject"] "Newsletter" {
  redirect "demo2@test.local";
}

I'm sending a message from demo3@test.local to demo@test.local with the 
subject "Newsletter". The message is delivered in the inbox of the 
original recipient and not forwarded.

Here's another sieve script:

redirect "demo2@test.local";
stop;

The same message is now both delivered in the original inbox and 
forwarded to demo2. It looks like this time the filter action was at 
least considered, but the implicit keep should not be effective, but is.

The Exim version is 4.93 on Ubuntu Linux 20.04.

This is a part of my Exim config. All addresses with maildirs and sieve 
data is stored in a database.

SQL_FILTER = \
        select E'# Sieve filter\n' || sieve_filter \
        from mail_entries \
        where local = '${quote_pgsql:$local_part}' and domain = '${quote_pgsql:$domain}' 
and maildir <> '' and sieve_filter <> ''
>
begin routers
>
virtual_user_filter:
        driver = redirect
        allow_fail
        allow_defer
        allow_filter
        user = Debian-exim
        data = ${lookup pgsql{SQL_FILTER}{$value}}
        address_data = ${lookup pgsql{SQL_QUOTA}{$value}fail}
        file_transport = address_directory

begin transports

address_directory:
        driver = appendfile
        directory = ${if eq{$address_file}{inbox} \
                {${lookup pgsql{SQL_MAILDIR}{$value}fail}} \
                {${if eq{${substr_0_1:$address_file}}{/} \
                        {$address_file} \
                        {${lookup 
pgsql{SQL_MAILDIR}{$value}fail}/.$address_file} \
                }} \
        }
        user = mail
        maildir_format
        delivery_date_add
        envelope_to_add
        return_path_add

The mainlog doesn't show more than the in and out events. There are 
especially no errors. (I can have errors when I leave out the Sieve 
filter comment prefix or use an invalid database query or break the 
database permissions. I've seen all of this but they're fixed and the 
messages are gone.)

How can I debug or fix the Sieve filter in Exim? I thought that the 
config I have here is about the same as what's already working with the 
Exim from Ubuntu 16.04.

-Yves

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