I'm guessing the reason for that is simply because WorldEdit doesn't know what a fluidstate is. Honestly, I don't know how to fix this issue aside from making a fork of WorldEdit or using mixins to make it recognize and be able to edit fluidstates.

I have the same issue - server crashed because of "Encountered an unexpected exception".
Could you please add that incompatibility to the mod description or will it be solved soon?
And thanks for this great mod!