DKIM not working - Mails do not get signed

PROBLEM DESCRIPTION

For any given domain, in Mail Settings, "Use DKIM spam protection system to sign outgoing email messages" is checked enabled, but in some circumstances outbound email messages are not signed from said domain. It's necessary to switch DKIM off and on again  to get it to work. 

Unable to send dkim headers

Troubleshoot log /var/log/rspamd/rspamd.log
Prompt for permission issues:
dkim_module_load_key_format: cannot load dkim key /www/server/dkim/xxxx.com/default.private: cannot map key file: '/www/server/dkim/xxxx.com/default.private' Permission denied

Set 640 to 644, then restart rspamd to send emails to view email source code
chmod 644 /www/server/dkim/xxxx.com/default.private
/etc/init.d/rspamd restart