The fix done for PHPBB-13842 causes the following error in IIS 8.5:
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: Section or group name 'system.webServer' is already defined. Updates to this may only occur at the configuration level where it is defined.
Oddly enough I can still navigate around the actual PHP files and can actually get to the installation screens. This error only seems to come up when bringing up the root of the site.