FML log: http://pastebin.com/G3SVAAj9

Reproduction:
Install Natura and FMP on Forge .953 for 1.6.4.
Create a new world and build a nether portal. (Creative works.)
Travel to the nether.
Return to the overworld.
Minecraft crashes.

No other mods need to be installed, and no other mods that I have used seem to cause this problem (including Mystcraft, my initial suspect, or Project Red which is the reason I installed FMP .212 to begin with).

Well that's interesting… we've been having a similar, possibly related issue on FC2 but we never suspected it may be Natura+FMP… I'll have a look into this, thanks!

@Tallinu Could you upload a zip/tar.gz of your world to somewhere and give us a link, if you still have it? Configs would be handy, too

Awesome, thanks. We'll take a look at it.

Sure, I'd been keeping backups and when testing I used a separate instance of minecraft and copied the world over each time I'd added or removed a handful of mods to avoid losing blocks between tests. I'm including config files for all the mods I've been using, as well as the backup save that I was testing with.

https://dl.dropboxusercontent.com/u/203721/Natura%20FMP%20Crash%20Save%20and%20Configs.zip

Hope this helps you guys on Forgecraft out! I always like watching DW20 on there, so I'd heard of the issue, but didn't realize it was still unsolved!

This should be fixed now.

What exactly fixed this? A newer FMP?

A change in where we perform actions after a chunk has loaded. It was a fault in the retrogen system.