We are receiving the error in the picture, but are not receiving the field, nor is any script showing an error. How can we determine which field is incorrect?

This is kinda strange since according to the error codes documentation the INVALID_FLD_VALUE error should have that info in message:

You have entered an Invalid Field Value {1} for the following field: {2}

If I were in your shoes I would try try to check all date values that you are trying to set for the wrong format. If it won't help I would try to set date field one by one until the error would be reproduced.