Did you try

T

and

F

?

no i didn't try that. yes/no ; 'T'/'F'; true/false. Which is it with a checkbox? Depending where I look I get a different answer.

I think true/false works in SS 2.0 ( as well as 1 and 0 because it's boolean ), and I think T / F work in SS 1.0

SS 2.0 here. always. also surprising that the error would be unknown if it's just a datatype issue

NetSuite is somewhat inconsistent. I believe the underlying value in the database may be 'T' or 'F'. Even in SS2.0 you get 'T' or 'F' when you run a search and you need to use 'T' or 'F' to set the default value of a fields that you create and add to forms / suitelets. Because the return of a workflow action script provides its value to something that isn't SS2.0, I would bet that you need to return 'T' or 'F'.