Thank you for your post.
Although I have not encountered this issue, talking with our telephone Support Technicians, one of them encountered a similar issue. After finding the case, the cause was a combination firewall and anti-virus application. Therefore, check your server for any anti-virus software, and also if a firewall is currently set up, temporarily turn it off and reinstall FileMaker Server. Once you are deployed, you can then reset the firewall. FileMaker, Inc. does not recommend running any anti-virus software on the server.