No idea why this crashes, but it does. And only the elven mana spreader it seems, no other axe I've seen does it. Crash report: http://pastebin.com/Qg4uyyms

Have you also reported this to Vazkii on the Botania Github?

Yeah, @Vazkii is lacking a null check there, though admittedly that tileentity should not be null.
I think I messed up the order of harvestBlock and onBlockHarvested.
I'll check it out.