Every 5 Hours an Error accurs that lags the Server (on Ubunto).

For what it's worth, it looks like somebody tried to connect to your server with a Vanilla client (which obviously won't work).
Is your server open to the general public or does it use a whitelist?
If it uses a whitelist, did the above connection attempt come from a whitelisted user?

I don't know why or how it is that every 5 hours someone was trying to connect to your server with a vanilla client, but this is an issue with minecraft/forge in general and I have no idea if they can or will do anything about it. Basically, every time a client tries to connect, forge server tries to check that they're running all the right mods. Each time a mismatch occurs, it adds lag time during this process. This pack doesn't really have many mods compared to many packs out there, but it's apparently enough that when a full vanilla client tries to connect it's lagging you. In any case, there's nothing we can do with this. If anyone can, it'd be Forge or Mojang, but I doubt anyone can prevent this, they have to run the check to ensure compatibility or much worse things can happen.