Thank you for posting.
Are all of the clients on the same subnet? Do the two affected clients have any firewalls or network security software that could be blocking outgoing connections? If you share a file using FileMaker Pro can the other clients connect to it?
This behaviour could indicate in issue with the server.pem file located right in the FileMaker Pro installation directory. Do these files exist on each client and appear to be about 6-8 KB in size? If this is not the case on some of the clients, it may be possible to use the server.pem from the clients that can connect.
server.pem is dead on ... the server.pem file did not exist. I found another thread on here where someone linked a self-extracting version of both .pem files which I downloaded and then was able to connect.
Any idea why the server.pem file does not install on some PCs? I have uninstalled/reinstalled FMP at least twice without it installing on this PC. We need to push this solution out to 5-10 more users, so if we can avoid having to double check that the server.pem file installs, it will save a lot of frustration.