I get that error message while moving my Structured Discussion page to wikitext on fr.wp [WhaRewpAMEwAAIu0UqAAAAAE] 2017-11-23 09:14:35: Fatal exception of type "Flow\Exception\InvalidDataException". I'm trying it because two users report that moving back to wikitext is not possible for them.

I don't have that problem on en.beta.wiki or ca.wp where I've tried it.

It may be an invalid task due to a one-time error.