So.... I'm having some really weird behavior here.
That is a small Site Workflow I have created to isolate the problem.
Both "Create Item" actions create an item in the same custom list with just the Title field filled in.
The second action fails with a generic "Failed to create item. Invalid filename" error.
To add to the oddness, the action only errors if the list is a custom list or document library, not on the default libraries.
Now, why does it fail after changing state, and only on custom lists? I don't see what I am doing wrong here...
This is Nintex Workflow 2010.