Does a Kaffarah become incumbent upon me for refraining from transferring embryos in my previous work in the field of IVF, without decisive proof of error, and this out of caution?

What the questioner mentioned does not necessitate a ruling of error or expiation, for a mere assumption does not entail a ruling. The original state is one of innocence, especially since she has been careful and cautious. She is advised not to open the door to intrusive thoughts for herself, due to the anxiety and distress they cause.