I have one site where backup/restore did not work. However, it is only on one site, I have tested the same example on some others where I have access and the bug did not occur. The problematic site runs under Linux, Mysql 4.1.10, php 5.0.4, safe mode off.
The bug does not occur if I replace relative path in require_once for absolute path:
restore: moodle/backup/bb/restore_bb.php, line 6:
backup/delete: moodle/mod/quiz/lib.php, line 290:
Everything other works as expected, but it might be that I have not yet checked all moodle features and that the issue will come again. I suspect that it may be a bug of php, but I cannot confirm it.