Server Help (Server Connectivity Issue) Unable to connect.

In the last 24 hours of real-time, a few friends and I have been playing on a friend's public, modded SB Server. The settings are all properly done, port-forwarded, along with a strong host. However, this morning at 1AM, the server was left on and running, but six minutes later from that, the server shuts down along with the host user's connection to Steam. We talked to the owner that morning, and he explained that his computer had self-updated with an automatic restart. We attempted to connect to the server again, however it is not working. The CMD works fine along with everything else, including no mod conflictions; NOBODY, and I MEAN nobody, is able to connect to it. Due to the self-update while the owner was asleep, this caused the server to be shut down suddenly/abruptly. Singleplayer works just fine. We are also unable to locate the issue's source; therefore, we are NOT able to find any corrupted files. There was also a fake troll "rollback" on the SB Launcher saying we went from Update 3 to Update 1 just days after the human sprites were "edited" without mods changing that. We believe mostly that this may be a universe file issue, but have not attempted to re-write (delete the universe files and re-run the game) the .universe files themselves yet. Referencing earlier, when I mentioned "NOBODY, and I MEAN nobody, is able to connect to it.", I meant that even the owner is not able to connect to his own server. Has such a thing like this happened before? Please comment your ideas, solutions, or perhaps what things you suggest for us. Universe rewrite is already listed, so don't pick that unless you can confirm it will work.

Also, I'm not the person who owns the server, but rather the guy who manages the modpack.
By the way, it only gives us the "Failed to connect to <IP & port>!" error.
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Update: deleting the universe files did not work, and we have confirmed to NOT have a rogue universe_lock file.

 

Can the host connect to the server locally? If not, that would be the place to start. They should be able to connect to the server using the server computer's IP address.

If that works, then the next place to look is to make sure the forwards are correct. Recreating them would be most effective in ensuring they didn't get borked.
If he is unable to connect locally, then there is likely an issue with the computer itself.

 

@Iazarus78
"Can the host connect to the server locally? If not, that would be the place to start. They should be able to connect to the server using the server computer's IP address."

Actually, we've attempted a series of events.
Our main issue was actually "No connection could be made because the target machine actively refuesd it.", so I did quite a haul of research. It turns out it was the owner's router. It was essentially taking a massive pigeon dump on him, so I had him reboot the Wi-Fi host. It freaking worked. We appreciate the help, however. After this reply is sent, I will be locking this as an archive. If anyone comes across this kind of problem again, direct them here.

By the way, it seems the error also targeted v.2.6.1 of Assorted Crafting Things as well with a (JsonPatchException) error too. This got resolved the same way we fixed the server. If you want to join the server full of custom content, add me on Steam! http://steamcommunity.com/profiles/76561198057093263/
My profile is set to Public, so you can type your name as a public comment there so I can recognize you. Our modpack is in my description. As for the IP, just ask! Again, thank you for attempting to assist us despite making this work ourselves.

I can't lock the post, but hey, it still works!