I had a similar case, when a text field with a large number of spaces (in the source) participated in the load. Then the application of function Trim() for this field helped.
Most likely, the load is affected most by one or more fields. They need to be found. Try not to load all the fields at once, but gradually adding their number. This will detect the field / fields that have a bad effect on the load.
When the "bad" fields are found, even then you can try to understand what is in them prevents normal loading.