Hi Martha,
As per my understanding, there might be a difference of encoding so file is getting failed via Data Import Monitor but importing successfully manually.
Please check the Encoding on the server that is generating the file, the encoding must be same as that of Sourcing server. In the past we have also faced the same issue and was resolved once the encoding of source System was changed to that of Sourcing.
Hope it helps!
Best,
Kushagra A