Logs: http://pastebin.com/MxFvD7Dx

Our plugin Source code: https://github.com/Armarr/Autorank-2

https://github.com/Armarr/Autorank-2/blob/master/src/me/armar/plugins/autorank/playerchecker/result/RankChangeResult.java

You're trying to add a player to a group, but passing the group as null. How is that supposed to work?

not sure where you're getting the replacegroup method from. There is no such method in Vault's perm handler, it looks like you're trying to perform:

(addGroup(player, world, newGroup) && removeGroup(player, world, oldGroup));

GM supplied the 'proper' way to add Groups in Vault. please check with them as the error is happening inside GM. It's erroring on grabbing the GroupName, so you might want to verify that you're not trying to add a null group. to the player.