Ho provato lo stesso codice con lo stesso db etc...su pc al posto che su mac e mi da questo errore su "riservato.php"

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Warning: open(/tmp\sess_5af1415cf6be1dceee93e6f72b507cdb, O_RDWR) failed: m (2) in c:/programmi/abria merlin/apache/htdocs/prova/admin/riservato.php on line 6

Warning: Cannot add header information - headers already sent by (output started at c:/programmi/abria merlin/apache/htdocs/prova/admin/riservato.php:6) in c:/programmi/abria merlin/apache/htdocs/prova/admin/riservato.php on line 17
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in linea 6 c'è:
session_start();

e in linea 17 c'è:
header("Location: $FF_authFailedURL");

help me, please...