I have also tried the following array solution but this didnt work either.
I had the same issue as @dtx211. I solved the problem by passing an array with the filters:
add_filter(‘acf/upload_prefilter/name=my_acf_upload_fieldname’, array( $this, ‘field_name_upload_prefilter’ ) );
AND
add_filter(‘upload_dir’, array( $this, ‘field_name_upload_dir’ ) );
Also ensure, that the upload directory has the right access rights and maybe already exists.