Hmm it’s hard to guess without seeing what’s the actual error. But if it works in “test mode” (“dry run”), it probably means the plugin works as expected, but there are some general issues with writing to the database.
Because the only additional actions that happen (when “dry run” is unchecked) are calls to these 3 native WordPress functions (which also happen when you import a file in Media Library):
wp_insert_attachment wp_generate_attachment_metadatawp_update_attachment_metadata
One of the common issues people had was trying to import files containing special characters – but without their database collation allowing special characters.
But that’s just one example, it can be anything.
