In an otherwise perfectly working setup, setting DEVELOPER_MODE to false breaks sabredav.
When the dav URL is accessed, the following is printed in the browser:

<d:error><s:sabredav-version>3.2.2</s:sabredav-version><s:exception>Sabre\DAV\Exception\NotImplemented</s:exception> <s:message>There was no plugin in the system that was willing to handle this GET method. Enable the Browser plugin to get a better result here.</s:message></d:error>