It could be...
Well, i'll recomend you, to try
system() or passthru() functions instead of exec()

also the problem could be solved, if you will change web server, or switch php installation from CGI to SAPI, or from SAPI to CGI (and try all 3 functions)

and also. this problem is known bug - so, you could try to install newer (or older) php version
http://www.experts-exchange.com/Web/..._20445300.html

se leggi sotto vedrai che uno degli utenti risolve il problema installando un pacchetto pre pronto (che per l'appunto) monta apache e quindi il problema si risolve...cmq metti anche una ver di php molto recente...tipo la 4.3.2 o superiori