http://pastebin.com/hHU3Uc8y

CyclopsCore-1.9-0.5.7.jar
EvilCraft-1.9-0.9.6.jar
Forge 12.16.0.1865

Hmm, I never actually went into the nether in 1.9. Thanks for reporting this!
In the meantime, disabling particles should stop the crash from occuring.

It's probably just an entity walking over a nether fish spawn block.
(I meant disabling MC particles, I'm not sure where you found this B:particle option)

"degradation effect" {
B:biome=true
B:knockbackDistort=true
B:mobSpawn=true
B:nauseate=true
B:paling=true
B:particle=false <<-----
B:sound=true
B:terraform=false
}

And I set video particles to ... oh decreased.. will try on minimal

Ah yes, that option has nothing to do with those particles :-)
This option only controls whether or not the Environmental Accumulator should show particles when degrading the environment.

I set B:particle=false and it still crashes.

It's weird though, its like it waits for a certain thing to happen, I just spent over 30 mins in the nether mining np and on the way out, boom crash. So is something occuring at random intervals to do with particles?

Don't know if it helps but this is what the server does http://i.imgur.com/N6joXr9.png

http://pastebin.com/kgVt5DTF

Having the crash as well in 1.9 in the nether.

@ProsperCraft If you set minecraft video settings of particles to minimum it stops the crash whilst ruben fixes the problem.