After installing the Security Update for Microsoft SharePoint 2013 ( KB2881078 ), we realized that the InfoPath forms stopped working.
The below error page appeared when we tried to open the InfoPath forms.
I checked the SharePoint logs and found the below error:
Exception Message: The type initializer for 'Microsoft.Office.InfoPath.Server.Util.UrlManager' threw an exception. Requested registry access is not allowed
So there should be an issue with the Application pool Account permission.
I run regedit > open HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\Office Server\15.0 > Assign read permission to the.App Pool account.
Now Reset IIS and then open the Page which has an InfoPath form. It is working !!!!!
If you don't know how to find the SharePoint App Pool Account, Open IIS and under the site, select the Web Application > Click on Basic Settings . > see what the APP Pool is
Hope it helps,