I was making changes that shouldn't have affected this, but... A two table browse window that was working yesterday just stopped working - well, throwing the titular error any time a change is entered.

SELECT Attend
SET FILTER TO TermID=m.TermID AND ClassID=m.ClassID

SELECT Last, First, EnrollID FROM Enroll INTO CURSOR Enr ORDER BY 1,2 
SET RELATION TO EnrollID into Attend

BROWSE FOR !EMPTY(Attend.EnrollID) FIELDS Last, First, Mon, Tue, Wed, Thu, Fri FREEZE &DayOfWeek

So it's sitting with the ENR cursor active, while attempting to update ATTEND. This code's copied from elsewhere, and the original still works. Attempting to recopy and tweak has not solved the issue.