Got this error - any clues how to debug?

Drupal 6.16. When importing a single image I get the following immediately:

An error occurred. /batch?id=28&op=do

The error page says:
# An error occurred while processing _imagefield_import_batch_process_multiple with arguments: Array ( [0] => sites/default/files/images/import/IMG_0443.png [1] => Array ( [title] => IMG_0443 [body] => [field_image] => Array ( [0] => Array ( [fid] => 0 [list] => 1 [data] => Array ( [description] => [alt] => [title] => ) [alt] => [title] => [filepath] => [filename] => [filemime] => [filesize] => 0 [_weight] => 0 [filefield_upload] => Upload [filefield_remove] => Remove [UPLOAD_IDENTIFIER] => 24e819e695355816dace61265d6155e1 [upload] => ) [1] => Array ( [fid] => 0 [list] => 1 [data] => Array ( [description] => [alt] => [title] => ) [alt] => [title] => [filepath] => [filename] => [filemime] => [filesize] => 0 [_weight] => 1 [filefield_upload] => Upload [filefield_remove] => Remove [UPLOAD_IDENTIFIER] => 5943af8376a2dc0a4330f34645d937a2 [upload] => ) [2] => Array ( [fid] => 0 [list] => 1 [data] => Array ( [description] => [alt] => [title] => ) [alt] => [title] => [filepath] => [filename] => [filemime] => [filesize] => 0 [_weight] => 2 [filefield_upload] => Upload [filefield_remove] => Remove [UPLOAD_IDENTIFIER] => 21cbc2b9cb7600341aede95880cbdd8f [upload] => ) [3] => Array ( [fid] => 0 [list] => 1 [data] => Array ( [description] => [alt] => [title] => ) [alt] => [title] => [filepath] => [filename] => [filemime] => [filesize] => 0 [_weight] => 3 [filefield_upload] => Upload [filefield_remove] => Remove [UPLOAD_IDENTIFIER] => b86091969ac87a70308b4964ef3ff914 [upload] => ) ) [node_status] => ) )