I solved the problem by fluke, without understanding what is really happening. In the original schema definition, specimenrequirements table primary key m_requirementid is not defined with "NOT NULL". It was added as a constraint implicit with the primary key definition. When I went back to redefine the database table to include "NOT NULL" explicitly, I was able to make the LabKey.query.insertRows call work. The similar problem in the editorGrid's add record button also went away.

I will close this issue for now but I would love to hear if anyone has a good explanation for this strange behavior.