PDF on FileMaker Server 14 (Windows) generating security error

I am experiencing a nasty problem with a Filemaker Server 14.0.4.412 on Windows Server 2012R2. PDF s in a container field don't display, but instead either one of two things happen:

1 on FMPA14 running on Terminal Server (Windows), an error displays 'Security settings do not allow downloading this file' (or something similar, the message is in Dutch). the dialog doesn't display a FileMaker icon, only a yellow warning sign (Windows?). The Terminal server has Acrobat Reader DC installed, which can open the PDF locally. Opening the FMP database with the FMPA locally displays the PDF, both stroed 'in file' and as Remote Container.

2 on FMP14 running on a normal Windows workstation, the PDF is opened in Adobe Acrobat in a separate window instead of being displayed in the container field.

We checked ports 80, 443, 16000 and 16001 being blocked on the Firewall (clients and server are within the same LAN). PNG and JPG will display OK, and the PDFs are stored properly, so it seems.

Where is this security error coming from? 

Could it be a problem with Windows Server 2012 not allowing Tomcat Server to run? And if so, what can I do to fix this?