Hi,
looks like the proxying Apache can't find a name for itself. Does your proxy vhost setting contains any ServerName/ServerAlias for your URL context? How did you set up your proxy?
I have no issues with these settings:
NameVirtualHost *:80 <VirtualHost *:80> ServerName public-fqdn ProxyPass /roundcubemail/ https://internal-fqdn/roundcubemail/ ProxyPassReverse /roundcubemail/ https://internal-fqdn/roundcubemail/
</VirtualHost>
-Martin