A server lockup the server won't crash but will become unplayable due to timeout.
Its a random chance when using IR Rolling stock in any way.

here is the log file
[https://cdn.discordapp.com/attachments/717929258042785803/731763488568901712/latest.zip]

This also drops anyone else on the server to 1fps. Even after the player that stated it leaves disconnects. Then everyone gets timed out until sever restart.

I had this issue when using jars such as Mohist and Magma which modify player movement code or something like that. Using plain old Forge gives me no issues. (Although EXTREMELY hard to configure stuff and just overall a pain)

Mod conflict, but may be fixed in the current test build and possibly next release.