Support for the current latest minecraft version was added today. So if you are running the latest version you should be able to join with no issues. Please feel free to post any issues in the comments. Thanks guys for your patience.
here's a nice way to get false banned, go in 1.12.2 and go to any kitpvp server, hold a sword and right click. then try to walk. you will trigger the anticheat (NoCheatPlus) for MOVING_SURVIVALFLY. i wish the staff would fix this and add a slowing effect as it could get newer players falsely banned. it's also very annoying