Thank you for your post.
FileMaker Server requires ports 80 and 443 to be available, even if Web Publishing is disabled. For more information, see Knowledge Base Article #13312 at:
Thanks for the link to that helpful article. That's the information I was missing and hadn't found on my own. I understand now that FMPro needs ports 80 and 443 for it's own internal users. We reconfigured our Rumpus server to allow http access using port 8080. Now, Rumpus and FMPro are playing together in the same sandbox.