Server-freeze with stack trace after restart and first connection

[František Novosad]

Describe the issue
This issue is pretty weird. The server freezes and prints stack traces that server not responded for X seconds. This only happens with SWM installed, otherwise

Spigot version

Plugin version
2.2.1 - built from dev git channel

How to reproduce
Not really sure, if I delete my overworld, it works for the first time, after another restart, the issue is back.

Crash reports (if available)
https://pastebin.com/M43NNcBn