Apache 2.4.3 (or maybe slightly earlier) added a new security feature that often results in this error. You would also see a log message of the form “client denied by server configuration”. The feature is requiring a user identity to access a directory. It is turned on by DEFAULT in the httpd.conf that ships with Apache. You can see the enabling of the feature with the directive
Require all denied
This basically says to deny access to all users. To fix this problem, either remove the denied directive (or much better) add the following directive to the directories you want to grant access to:
Require all granted
as in
<Directory "your directory here">
Order allow,deny
Allow from all
# New directive needed in Apache 2.4.3:
Require all granted
</Directory>