It could be that you have an attachment post with an empty file name. Perhaps due to faulty imports.
Edit file /wp-content/plugins/wpsolr-pro/wpsolr/core/classes/engines/class-wpsolr-abstractindexclient.php:
Replace
$response = $this->search_engine_client_extract_document_content( $post_attachement_file );
with
if ( empty( trim( $post_attachement_file ) ) ) {
throw new \Exception( 'Attachment without empty file name.' );
}
$response = $this->search_engine_client_extract_document_content( $post_attachement_file );
You will then see the attachment ID in the error message.