After much troubleshooting, I was able to find the cause of the error and solve the problem. It turns out that this message had nothing to do with importing yes/no fields, but was rather related to a problematic business rule.
If ever you have a similar problem, here are the steps to solve it:
1. Start by checking your business rules for the relevant entity.
2. Pay particular attention to any conditions including numeric comparisons for fields that are imported using the dataflow, such as FieldA is greater than X.
3. If, when importing this field, it sometimes includes null values, you will receive this error in your dataflow log and the record upload will fail.
4. To solve this problem, add a new condition before your numerical comparison condition in your business rule. The new condition should be as follows : IF FieldA Does not Contain Data.
5. Place your previous numerical comparison condition under the FALSE branch of the newly created IF FieldA Does not Contain Data condition.
6. Refresh your dataflow again.
7. Rejoice.
Basically, the "String was not recognized as a valid Boolean" error refers to the fact that the business rule was unable to get a valid TRUE/FALSE value from the numerical comparison of a null value. Indeed, IF null < 3 = error.
Hope this explanation can help others!
