Somebody tried to connect to the server (Waterfall) and I got this error in console:

java.util.concurrent.CompletionException: java.lang.NullPointerException
at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1592)
at net.md_5.bungee.scheduler.BungeeTask.run(BungeeTask.java:63)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
at me.lucko.luckperms.common.event.EventFactory.handleUserLoad(EventFactory.java:207)
at me.lucko.luckperms.common.storage.AbstractStorage.lambda$loadUser$2(AbstractStorage.java:107)
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
... 4 more

Can you update to the latest version & try again.