(v0.51) - The 'Reclamation' quest (Iron Harvest) requires you to find some Nether Magnesium Ore to complete. However, according to the NetherOres\common.cfg file, it's disabled.
Code:
magnesium {
I:BlocksPerGroup=5
# Disables generation of Magnesium (overrides global ForceOreSpawn)
B:Disable=true
# Force Magnesium to generate (overrides Disable)
B:Force=false
I:GroupsPerChunk=4
I:MaxY=127
I:MinY=1
# The pickaxe level required to mine Magnesium
I:MiningLevel=2
# Retroactively generate Magnesium if enabled in CoFHCore
B:Retrogen=true
}