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… Continue reading