It's not so much that the Tomcat server is not allowed to run, it is that the system settings on the TS box don't allow content from it to be displayed for security reasons.
If the port was blocked you would get a different message or no message at all.
What is the full OS on the TS machine?
The OS on the TS machine is Windows Server 2012 R2 Standard (64 bits, Dutch language).
The system has 8 Gbyte RAM.
On the client machine (case 2, opening the PDF in acrobat) the Windows version is Windows 7 Professional.
I can't check the security settings right now, because I do not have an administrator key.
Are the two different 'non display' behaviors related to the same cause: security settings on the TS settings, and user profile, respectively?
Ah! Problem solved. Thanks for the feedback, Wim.
It was caused by the security settings of Windows Explorer. You need to 'trust' content from the built-in webserver that serves the content.
By the way: be aware that installing a plugin from a container field is also effected by this security setting.