There is something wrong, and I have 6 crash reports to prove it:

• Using ATG, Fastcraft, and a lot of other mods
• Same as before
• Again
• This time without Fastcraft
• Using neither ATG nor Fastcraft
• Fresh instance with only Botania and Baubles (that one took its time to crash, but in the end ...)

Now, they all contain the following lines:

[...]
at vazkii.botania.common.block.BlockModFlower.func_149853_b(BlockModFlower.java:151)
at vazkii.botania.common.core.handler.BiomeDecorationHandler.onWorldDecoration(BiomeDecorationHandler.java:53)
[...]

I guess somehow placing a Tall Mystical Flower failed, or rather shouldn't happen in the first place?

In all tests I was flying around generating terrain, using Botania version r1.6-183 with slightly tweaked configs:

I:worldgen.flower.patchChance=8
I:worldgen.flower.patchSize=8
I:worldgen.flower.quantity=2
D:worldgen.flower.tallChance=0.15