I recently applied the December Cumulative update to a SharePoint 2013 on-premises server (PSCONFIG was successfully run post update) and now suddenly constantly receive a 404 when navigating to a site that has both Windows and Forms authentication, the 404 happens when choosing "Forms Auth" from the drop down box (Windows Authentication works fine).
I've tried deleting and extending the web application, same behavior.
I've tried creating a custom login page and setting the app to use the custom page, same behavior.
Fiddler tells me the 404 is on /_Forms/Default.aspx though I've confirmed this file exists in directory where the website files are stored, and the FORMS directory in \14\TEMPLATE\IDENTITYMODEL\and \15\TEMPLATE\IDENTITYMODEL\both seem fine (each contain a Default.aspx and web.config).
Any thoughts on what could be causing this?