https://cdn.discordapp.com/attachments/214104876907495425/457037966796128258/serverguicrash.txt

Looks like something is trying to open a GUI on the server side of a headless server. @Divineaspect might know more.

Try updating VanillaFix. You're running an outdated version from 1.0.5 when we already working on stuff for 1.0.9, update to the current CurseForge release and see what happens after.

Let me check to see if we've updated, then I'll get back to you.