When calling "MetaUtils.getPrefix(new GroupLink(group), null, null, true);"
I got a Unsupported Operation Exception

When i trace it down, it comes down to this,
You create here a new Empty list, but that list is immutable
Documentation:

Returns an empty list (immutable).

https://github.com/lucko/LuckPerms/blob/2429831153d574da42bd9d290691294d57ec590c/common/src/main/java/me/lucko/luckperms/api/implementation/internal/PermissionHolderLink.java#L68

So when you do later the add method on it, it is unsupported :/

Stacktrace
http://pastebin.com/3vbjWNFs

http://pastebin.com/5BJLbqFG
full stacktrace, dint see reason for it, but here ya go ;)

Yeah by bad, didn't see what you meant initially.