Found the problem thanks to the suggestion from @sarnold. My Application depends on mpm_prefork
and up till Ubuntu 13.04, this module was automatically enabled when the apache2-mpm-prefork
package is installed. I assumed this was still the case, but it turned out that it was running mpm_event
.
It seems that in Apache 2.4 the packaging of MPM's has changed and mpm_prefork
needs to be enabled manually after installation:
sudo a2dismod mpm_event
sudo a2enmod mpm_prefork
sudo service apache2 restart
Now the problems seem to have disappeared.