Ok, just installed January 2018 CU on SharePoint 2013 farm which was not patched since SP1 before that. It seems the bug from one of the security updates introduced in March 2015 is still there and ruins InfoPath forms.
There is a blog with a solution for that (thanks Kate). Looks like no one reported this to MS since this was not fixed, and now it’s probably too late.
I’ll repost the solution here in case the source will go down, looks like it was not updated for a while…
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