yeah... At least next time you'll know a bit more about what to look for.
I had a similar thing last week when one of my servers refused to speak to its HSM. Eventually I gave up and rebooted the thing, and then it started working again. So I have no idea what caused it and thus I can't prevent it from happening again.
Following this tutorial I tried to verify if the mod_security module was loaded on my Apache2.
apachectl -M | grep --color security
I got this error message:
AH00526: Syntax error on line 187 of /etc/modsecurity/modsecurity.conf:
ModSecurity: Failed to open the audit log file: /var/log/apache...
Normally, on Debian apache runs as the www-data user, not apache. The ownership of the file (root:adm) is correct, as are the perms, changing them would be wrong too so the problem lies elsewhere.