The code I am running :
bush = GameRegistry.registerBlock(new BlockBushes().func_149663_c(Strings.BUSH_NAME), ItemBlockMulti.class, Strings.BUSH_NAME, Reference.MOD_ID, BUSH_TYPES);
ItemBlockMulti extends ItemBlockWithMetadata, with a constructor using (Block, ArrayList<String>), and BUSH_TYPES is of type ArrayList<String>, by the way.
It doesn't matter actually, if commented out, it crashes the same with the next block registration attempt.
The debug reached there:
It looks like the ItemBlock couldn't get the same id as its Block, for some reason ?
Note that the block id reached exactly 256, which may be related to that. (The previous block used blockId=255, and loaded correctly with itemId = 255)
Looking into itemRegistry didn't give me anything.