Bugzilla shows "internal error" after checksetup

After executing checksetup sometimes bugzilla is not available anymore but shows

The server encountered an internal error or misconfiguration and was unable to complete your request.

In the error log of the apache you will find something like

Permission denied at /usr/share/perl5/Bugzilla/Config.pm

Make sure that

 /etc/bugzilla3/params
 /etc/bugzilla3/localconfig

are readable and writable by www-data.

 chown :www-data /etc/bugzilla3/params /etc/bugzilla3/localconfig
 chmod g+rw /etc/bugzilla3/params /etc/bugzilla3/localconfig