Server lags and crashes when player joins.

When I run my server in these versions on my separate computer, it works fine except when a player joins the server. Any other versions before works completely fine. (It's been happening since 18w19a was released, but I can't select it.) I've tried to delete all server files, delete playerdata folder, delete my datapacks, switch OS versions (from Debian 7 to Ubuntu 18.04), delete server.properties, and even switch worlds. Nothing works at all. When I do join in these versions, the server lags behind in time and it will keep saying I am moving too quickly. Below is the crash report file from server files. I've always used 1-2gb RAM for my server to run.