This is similar to a problem I was having. When I tried to duplicate the problem in a simpler sheet it went away.
I was getting the TO field blank error, but when I looked in the preview, the field had data. I was using a fairly convoluted formula to populate the field. My suspicion is that it was taking too long to get the data into the field and Formmule was executing before it completed. I replaced the convoluted formula with a simpler one and (I had done this for a completely different reason, but none-the-less) this made the problem go away.