Well, 1&1 don't like me changing trust. This rasied a new exception I've not seen before:

Configuration Error

Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.

Parser Error Message: This configuration section cannot be used at this path.  This happens when the site administrator has locked access to this section using <location allowOverride="false"> from an inherited configuration file.