Thank you for your post.
When you launch FileMaker, it looks for the license key on the network to make sure the same license key is not being used elsewhere. In addition, all settings for web publishing and network sharing are set. When you quit FileMaker Pro, that information is released from the network. When you force quit, the network sharing and license key information eventually are dropped from the network, so you may get an error that the license key is still in use, but the port number remains locked up for some time. Try rebooting the machine after force quitting.