There is a bug in Magento that will crash the shop when a request is made to the API root URL. Magento versions up to 1.9.0.1 are affected. This was fixed earlier by Magento in patch SUPEE3762 (update: has since been replaced with SUPEE6788), but apparently many people have overlooked this one.

Symptoms:

The frontpage will redirect to /index.php/install

Pages will stay white

You will get errors such as:

PHP Warning: SoapServer::SoapServer(): I/O warning : failed to load external entity "http://hostname.com/index.php/api/soap/index/?wsdl=1&wsdl%2F_vti_bin%2FListData_svc%2F%24metadata=" in /var/www/html/lib/Zend/Soap/Server.php on line 811 1 2 3 PHP Warning : SoapServer :: SoapServer ( ) : I / O warning : failed to load external entity "http://hostname.com/index.php/api/soap/index/?wsdl=1&wsdl%2F_vti_bin%2FListData_svc%2F%24metadata=" in / var / www / html / lib / Zend / Soap / Server . php on line 811

A quick fix (thanks to Ben Lessani) is to put this on top of index.php:

PHP Warning: SoapServer::SoapServer(): I/O warning : failed to load external entity "http://hostname.com/index.php/api/soap/index/?wsdl=1&wsdl%2F_vti_bin%2FListData_svc%2F%24metadata=" in /var/www/html/lib/Zend/Soap/Server.php on line 811 1 2 3 PHP Warning : SoapServer :: SoapServer ( ) : I / O warning : failed to load external entity "http://hostname.com/index.php/api/soap/index/?wsdl=1&wsdl%2F_vti_bin%2FListData_svc%2F%24metadata=" in / var / www / html / lib / Zend / Soap / Server . php on line 811

Thanks to Miguel Balparda, the Magento Slack crew and ShirtsOfHolland for helping debugging this issue.