Well looks like today is the day of doom https://minecraft.net/en-us/article/update-aquatic-out-java

2 days before the British summer holidays start, good timing Mojang! (plebs).

For my sanity, and so I dont tear off heads, for all the annoying peolpe that spam “can u update teh server!!!” can you please direct them to this thread, if needed I will create a custom command to tell them why its going to take months.

1.13 Update news:

• The server software isn’t even close to ready! Spigot had its first 1.13 pre-release only 4 days ago so developers could start to see the mammoth changes, theres a lot of background stuff you dont see thats changed.
  https://www.spigotmc.org/threads/bukkit-craftbukkit-spigot-bungeecord-1-13-pre7-development-builds.328883/
• There are still a ton of game-breaking bugs that crash the client or kill you randomly with their 1.13 server https://github.com/SpigotMC/SpigotCraft/issues
• The plugin I use to allow newer clients to connect to the older server software is out! But there are some known major bugs with it, so im going to wait a week to see if they can be fixed first. Expect me to test it very soon to get those 1.13 clients connecting to 1.12, the downfall is it will break some plugins, Like anti-cheat. Rip.
• People really struggle to understand the amount of backend changes that have happened. This is the biggest minecraft update in history. So much has changed in the backend, all server software and plugins have to adapt. Let alone for our own specific setup. We will be loosing plugins.
• There are so many known issues at present, its not even worth testing yet, we will stay on a stable release as long as we can, my plan will be to add the client hack plugin to allow 1.13 clients connect to 1.12.2 server once its fixed up a bit.

For now just backdate your client to be 1.12.2.