Ultimate odd client crash

[Synasthesia]

I've been playing on a friend's Ultimate server for about 3 weeks now with no issues. Until yesterday.

I was in a mystcraft age, and used /home to return to my sethome spot. When I did this (reproducable, just not 100% of the time), my client crashes with a "Exception in World Tick" error. I scoured the log, and couldn't find out the exact issue, so I am reaching out in an attempt to remedy this issue. We have a couple other players that also have this issue, but they are just playing with the crash, instead of trying to fix it.

Pastebin: http://pastebin.com/FCnE3Cdd

In some testing, it seems that the chunks that I am attempting to teleport to need to be UNLOADED for this crash to occur. I haven't had a crash while the chunks were already loaded.

 

[GuildMaster]

Unfortunately, Mystcraft is buggy. There are a lot of problems with it + Chicken Chunks. Mystcrafts unloads/loads worlds incorrectly and this is why there are lots of problems.
Try updating your Java too. The crash report simply indicates bad coding.

 

[NdMSnIpEz]

Concurrent Modification Exception is an extremely common error with any kind of teleporting, especially with mystcraft. It has many causes - the main to do with Loaded and unloaded chunks.
Mystcraft isn't very smart about this, though. In 1.5, it seems to of improved, but as you mentioned, you're in ultimate pack, 1.4.
Some advice I can give is:
Inform players to spot load the /home chunks, and the chunk that they're leaving in the age (Have designated chunks for leaving).
Or disable mystcraft. Mystcraft is notorious for world leaks and lag, so I'd definitely reccomend this option to help with both problems.