My server is showing as online with the latest version of 1.12.2 paper. When I hover over the ping signal, it says “server out of date” I am on 1.12.2 and it worked fine last night before I powered down the server.
The Minecraft client (game) is newer than the server profile version. Either lower the game version or update the server to a newer profile.
They are both 1.12.2