Generic Possible Reasons (until someone who knows more can tell you):
It could be a versioning problem. Such that you need to update to the latest version to connect to their server, or the developer didn't update their server to allow the new updated version of the game. (A magic number is usually a hash value for integrity validation. It's not foolproof though.) Such an outcome can usually be patched quick enough if reported to the developer. Most Likely scenario.
It could mean that their game server is not available. Such that the developer didn't pay fees in time to the hosting provider, or that the server has been locked down by hosting admins in order to deal with an active hack elsewhere in their network. (yes, this happens.)
It could mean that your computer has a virus that has infected a file's magic number they are checking for. (Please NEVER disable anti-virus unless the vendor has legitimately earned your trust; say for example by following advice from your workplace's I.T. department or the source instructions is the same official one where you got your Operating System from and you can verify it the message comes from them.)
Opening ports is also risky, but absolutely necessary sometimes.
It could mean that the program has a glitch where your specific computer with your specific configuration results in a failed check despite the details being correct. Programmers are falliable. It happens. Such an outcome can usually be patched quick enough if reported to the developer. Also, check to see if you have a configuration file needing maintenance. It might simply be that.
It could mean that the program you were trying to run was identified as malware by the hosting provider; and as such they've quarantined new network connections, but haven't removed the files - a form of reverse-shadowban so as to track new activity by the hacker and learn more about them. Extremely rare.
It could also mean that your DNS has issues.
It could mean any of these or something completely different. Hope you get resolution soon!