If your proxy is has some error (like misconfigured sudoers, puppet not available, etc...) and you try to boot a host through unattended mode [1], the exception will not show up when you try to fetch the kickstart template. Instead, it will show something like No HTTP_REFERER was set in the request to this action, so redirect_to :back could not be called successfully.

The application controller should be aware of this situation and don't try to redirect to back in it, instead just render the error.

[1] an example of such error - ProxyAPI::ProxyException: ERF12-0104 [ProxyAPI::ProxyException]: Unable to set PuppetCA autosign for tonia-rippeon.lobatolan.home ([RestClient::NotAcceptable]: 406 Not Acceptable) for proxy https://centos7-devel.lobatolan.home:9090/puppet/ca