Problem creating deleted users on restore

I have a backup that I use for testing freshly installed sites.
The backup was originally taken in August 2010, and I have last successfully restored it in August 2011.
Today I've tried to restore it twice and both times have got the following:

error/unknown_context_mapping

More information about this error

Stack trace:
line 637 of /backup/util/dbops/restore_dbops.class.php: restore_dbops_exception thrown
line 869 of /backup/util/dbops/restore_dbops.class.php: call to restore_dbops::send_files_to_pool()
line 691 of /backup/moodle2/restore_stepslib.php: call to restore_dbops::create_included_users()
line 34 of /backup/util/plan/restore_execution_step.class.php: call to restore_create_included_users->define_execution()
line 153 of /backup/util/plan/base_task.class.php: call to restore_execution_step->execute()
line 148 of /backup/util/plan/base_plan.class.php: call to base_task->execute()
line 157 of /backup/util/plan/restore_plan.class.php: call to base_plan->execute()
line 302 of /backup/controller/restore_controller.class.php: call to restore_plan->execute()
line 144 of /backup/util/ui/restore_ui.class.php: call to restore_controller->execute_plan()
line 45 of /backup/restore.php: call to restore_ui->execute()
Output buffer: Notice: Trying to get property of non-object in /backup/util/dbops/restore_dbops.class.php on line 827