This mod crashes my server only after a first player joins with the Ticked Entity error.

Crash report here: https://pastebin.com/6xdHiz1G

Please report this to LLibrary. They are trying to get a capability without even checking that the entity has that capability. link

At minimum they definitely should NOT throw an exception if they aren't using Entity#hasCapability to first check. In this case, null is equivalent to a Entity#hasCapability check, but instead of logically terminating silently, they throw an exception. This is on their end.