I would not clean the ESM. That problem sometimes occurs when users forget to revert their esmified ESPs or converted-to-masters ESPs.
Yeah, I returned the original when that didn't solve anything.
Even if it would work after moving the esm in the load order (which I did to get more info if it was load-order dependent, then I could search for a fix there), it is not to get it to work that is most important but why. What is it that makes an esm behave badly in this manner? I just know there is a thing one shouldn't do when building esm because it can lead to just these things - and I can't remember!
I haven't bug hunted any more on this, but if someone knows the answer of have some ideas about issues around building esm and there troubles please post.
Cheers!