I thought the game bugged out at the character selection screen, but then I noticed the tiny buttons in the bottom right. The UI needs to be able to adjust accordingly for higher resolutions.

Apologies, you can change the UI size in the settings menu. Although the settings menu only appears on the login screen, or when you are already logged into the game. 

Because of the games pixel art nature, dynamically resizing the UI while keeping it looking good, is a bit of a challenge.