After upgrade to 3.10 we cannot duplicate quiz. We waited for MDL-70422 but this didn't solve the problem. The problem is only on one of our university portals, the oldest one (since moodle ages ago), upgraded to each new version without any problems so far.

What is "line ? of unknownfile: call to progressive_parser->end_tag()"

The error is 

Failed to find an answer matching "" in the question_answers database table. This occurred while restoring the question with id 82794 in the backup file, which has been matched to the existing question with id 82794 in the database.

The log stack is 

Error code: error_question_answers_missing_in_db

• line 199 of /backup/moodle2/restore_qtype_plugin.class.php: restore_step_exception thrown

• line 137 of /backup/util/plan/restore_structure_step.class.php: call to restore_qtype_plugin->process_question_answer()

• line 121 of /backup/util/helper/restore_structure_parser_processor.class.php: call to restore_structure_step->process()

• line 178 of /backup/util/xml/parser/processors/grouped_parser_processor.class.php: call to restore_structure_parser_processor->dispatch_chunk()

• line 109 of /backup/util/helper/restore_structure_parser_processor.class.php: call to grouped_parser_processor->postprocess_chunk()

• line 148 of /backup/util/xml/parser/processors/simplified_parser_processor.class.php: call to restore_structure_parser_processor->postprocess_chunk()

• line 92 of /backup/util/xml/parser/processors/progressive_parser_processor.class.php: call to simplified_parser_processor->process_chunk()

• line 190 of /backup/util/xml/parser/progressive_parser.class.php: call to progressive_parser_processor->receive_chunk()

• line 278 of /backup/util/xml/parser/progressive_parser.class.php: call to progressive_parser->publish()

• line ? of unknownfile: call to progressive_parser->end_tag()

• line 179 of /backup/util/xml/parser/progressive_parser.class.php: call to xml_parse()

• line 158 of /backup/util/xml/parser/progressive_parser.class.php: call to progressive_parser->parse()

• line 110 of /backup/util/plan/restore_structure_step.class.php: call to progressive_parser->process()

• line 181 of /backup/util/plan/base_task.class.php: call to restore_structure_step->execute()

• line 178 of /backup/util/plan/base_plan.class.php: call to base_task->execute()

• line 168 of /backup/util/plan/restore_plan.class.php: call to base_plan->execute()

• line 385 of /backup/controller/restore_controller.class.php: call to restore_plan->execute()

• line 3494 of /course/lib.php: call to restore_controller->execute_plan()

• line 3544 of /course/externallib.php: call to duplicate_module()

• line 261 of /lib/externallib.php: call to core_course_external::edit_module()

• line 81 of /lib/ajax/service.php: call to external_api::call_external_function()