Required dependencies
As for now xAuth needs the h2 library when running in non-mysql mode.
As it is not allowed to automatically download this file directly from an external site you have to do it manually.
- Open the following link in your favorite browser https://mvnrepository.com/artifact/com.h2database/h2/1.3.164
- Click the link "jar" which should download a file named h2-1.3.164.jar
- Place this file into your lib directory located in your minecraft servers root.
- Start / reload the server to enable xAuth
If you have followed this instructions the plugin will load without any SEVERE message.