Here you have an image with the error.
I figured it out how to fix this issue.
What was the fix?
For me, re-deploying the FileMaker Server helped. Not sure why the initial deployment did not work.
The only thing I know is that during the initial deployment I had both Ethernet and Airport connections active, so my computer had two different IP addresses.
I am wondering if it will stop working after I move to another network and my IP address changes. (This is a testing installation on my MacBook.)