Just upgraded my web server to the current version of ubuntu and got caught out by a change in apache httpd. (Note to self: RTF-release-notes next time.)
It resulted with a 403 / Forbidden error, with the browser showing
You don’t have permission to access / on this server, and a
client denied by server configuration entry in the log.
The issue was the addition of mod_authz. It needs a new entry in the Directory section of your web site config:
Require all granted
It is equivalent to the old allow from all. Check the Apache mod_authz_core page for more details.
Luckyily this time it was a fairly easy fix, and mod_authz seems to have some fairly interesting features to explore. In any case, if your web sites stop working after an upgrade in your ubuntu server, this is probably the issue.