[exim] DKIM: signing failed (RC -102)

Hi folks,

My first attempt to configure DKIM on a Debian wheezy box running Exim4
4.80 (exim4-daemon-heavy) keeps ending with this error:

DKIM: signing failed (RC -102)

It shows up in the Exim logs every time I try to send a message. As
expected, when the messages arrive they have no DKIM headers. My Debian
Exim configuration has these variables set:

DKIM_DOMAIN = umrk.nl
DKIM_SELECTOR = umrk
DKIM_PRIVATE_KEY = /etc/exim4/dkim.256-bit.private.key
DKIM_CANON = relaxed

A 256-bit key is weak, but for now I want to keep things simple. The key
file was created with this command:

~# openssl genrsa -out dkim.256-bit.private.key 256

AFAIK Exim uses those variables to create this configuration:

  remote_smtp:
    debug_print = "T: remote_smtp for $local_part@$domain"
    driver = smtp
    dkim_domain = umrk.nl
    dkim_selector = umrk
    dkim_private_key = /etc/exim4/dkim.256-bit.private.key
    dkim_canon = relaxed

The key file has permissions 640 with group Debian-exim. I tried putting
double quotes around the variables, but that made no difference. I saw
that the DKIM directives in /etc/exim4/conf.d/transport/30_exim4-
config_remote_smtp had not been indented properly, but after I corrected
that it made no difference either.

Can anyone say what might be causing the error in question?

Thanks,

Jaap