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Remove it as that error can be caused by optifine.
If you really have to use it then if chunk loading is set to multi-core change it to normal.
If you crash in the future then remove optifine first to see if that fixes it.
use the ftb launcher and grab the 2.5.3 server, which is a zip file. Now backup the whole server and then delete everything except the world save. Unzip and upload the new version and then play.
The growth pulsars go directly under the dirt and are stackable up to 64 but around 24 should be enough for a farming station to keep up.
You can use the lamp of growth and the watering can if you wish.
The mod was removed from the pack. There are other options like Growth Pulsars, Lamp of Growth & Watering Can. The reinforced watering can can be used in an autonomous activator as well. The growth pulsars stack as well up to 64 max.
You want to be using java 8 anyway if it is new packs you are playing.
If you want to play old packs and are using curse then change the java version in the curse minecraft options.