I have finally fixed this (I had used hiawathaproxy in the mean time).
There was a problem with the configuration file for one of my domains. I deleted the domain, and added it again in the kloxo web interface.
In the directory
/opt/configs/apache/conf/domains
I did:
grep SSLCertificateFile *
One of the domains only listed two lines, all the rest had three lines.
This link led me to look at this.
http://stackoverflow.com/questions/17898135/httpd-not-starting-after-installing-certificate6) Message "Server should be SSL-aware but has no certificate configured" in
error log
Since 2.2.12, Apache is stricter about certain misconfigurations concerning
name based SSL virtual hosts. See NEWS.Debian.gz for more details.
And NEWS says:
* The new support for TLS Server Name Indication added in 2.2.12 causes
Apache to be stricter about certain misconfigurations involving name
based SSL virtual hosts. This may result in Apache refusing to start
with the logged error message:
Server should be SSL-aware but has no certificate configured
[Hint: SSLCertificateFile]
Up to 2.2.11, Apache accepted configurations where the necessary SSL
configuration statements were included in the first (default)
<Virtualhost *:443> block but not in subsequent <Virtualhost *:443>
blocks. Starting with 2.2.12, every VirtualHost block used with SSL must
contain the SSLEngine, SSLCertificateFile, and SSLCertificateKeyFile
directives (SSLCertificateKeyFile is optional in some cases).
When you encounter the above problem, the output of the command
egrep -ir '^[^#]*(sslcertificate|sslengine|virtualhost)' \
/etc/apache2/*conf* /etc/apache2/*enabled
may be useful to determine which VirtualHost sections need to be changed.