Does this error appear when you open the world up to LAN, or is this is the error they get when trying to join?

When they are trying to join.

Could you attach their log file? (You will need to reproduce this error). Thanks

latest.log
Only happens when using Destoy mod, in the log its just disconnected.

The bug exists in version 0.3

You've probably already figured this out but it seems to be an issue relating to the ICU library

The reference to the ICU DateFormat definitely caused #102, and has been fixed in 0.4, by replacement of the class with the built-in java.util.DateFormat. @BowBoy1 I've not investigated this one extensively, why do you think that? It doesn't seem that that caused the crash in the log above. Hopefully you're right though, as that would mean it's already fixed.

My bad, I found this open issue through #106 and thought it was related to the issue I was having earlier. This one's still a pretty major concern though, it still breaks multiplayer. If I find any useful information on this problem I'll be sure to share it.

Fixed in 0.4. Or at least I can't replicate anymore...

Great, I can add it to the server of my friends and me.