Did more tests. Rolled back all Windows Updates on my Windows 8.1 workstation that were recently applied before June 10th. Rebooted, PDF Export started working under IIS. Added the updates back one by one, export failed after I added this update back. https://support.microsoft.com/en-us/kb/3057839. I went to one of the web servers and rolled KB3057839 back, rebooted and the PDF export works now as well.
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