Here's the Steps:

1. Backup the course in 1.9.
2. Go to the course in 2.3.
3. Select Restore from the Settings menu.
4. Add the .zip backup file exported from 1.9 using the file uploader and click Restore.
5. Receive the "not a standard Moodle file" message and click Continue.
6. Under "Restore into this course," choose "Merge the backup into this course" and click Continue.
7. Get the "error/file_invalid_path" message

PHP Error Message:

[05-Feb-2013 17:55:00 UTC] Default exception handler: error/file_invalid_path Debug:
Error code: file_invalid_path
$a contents: C:\moodledata/temp/backup/5e56cabee22590231f5520ba8e44f73d/course_files\Bentley_-_Culture_and_Kultur.pdf

• line 1215 of \backup\converter\moodle1\lib.php: moodle1_convert_exception thrown
• line 1302 of \backup\converter\moodle1\lib.php: call to moodle1_file_manager->migrate_file()
• line 578 of \backup\converter\moodle1\handlerlib.php: call to moodle1_file_manager->migrate_directory()
• line 566 of \backup\converter\moodle1\handlerlib.php: call to moodle1_files_handler->migrate_course_files()
• line 312 of \backup\converter\moodle1\handlerlib.php: call to moodle1_files_handler->process()
• line 371 of \backup\converter\moodle1\lib.php: call to moodle1_root_handler->on_root_element_start()
• line 756 of \backup\converter\moodle1\lib.php: call to moodle1_converter->path_start_reached()
• line 228 of \backup\util\xml\parser\processors\simplified_parser_processor.class.php: call to moodle1_parser_processor->notify_path_start()
• line 102 of \backup\util\xml\parser\processors\simplified_parser_processor.class.php: call to simplified_parser_processor->process_pending_startend_notifications()
• line 92 of \backup\util\xml\parser\processors\progressive_parser_processor.class.php: call to simplified_parser_processor->process_chunk()
• line 169 of \backup\util\xml\parser\progressive_parser.class.php: call to progressive_parser_processor->receive_chunk()
• line 212 of \backup\util\xml\parser\progressive_parser.class.php: call to progressive_parser->publish()
• line ? of unknownfile: call to progressive_parser->start_tag()
• line 158 of \backup\util\xml\parser\progressive_parser.class.php: call to xml_parse()
• line 137 of \backup\util\xml\parser\progressive_parser.class.php: call to progressive_parser->parse()
• line 150 of \backup\converter\moodle1\lib.php: call to progressive_parser->process()
• line 129 of \backup\converter\convertlib.php: call to moodle1_converter->execute()
• line 209 of \backup\util\helper\convert_helper.class.php: call to base_converter->convert()
• line 421 of \backup\controller\restore_controller.class.php: call to convert_helper::to_moodle2_format()
• line 35 of \backup\restore.php: call to restore_controller->convert()

[05-Feb-2013 17:55:01 UTC] Potential coding error - existing temptables found when disposing database. Must be dropped!