I found it! I turned on debug level logging in apache.
There are additional .htaccess files in the “app/assets” and “app/bundles” folders. The content of these files are not valid on apache 2.4.
“Invalid command ‘allow’, perhaps misspelled or defined by a module not included in the server configuration”
“Invalid command ‘Order’, perhaps misspelled or defined by a module not included in the server configuration”
The files need to be modified to take into account Apache 2.4.
For the moment, I have renamed the files “.htaccess-invalid” until I can come up with a proper 2.4 version.