360210 php flash problem on MAC

We have a file reference upload set up and working uploading to a PHP script on an apache server. It's a multiple upload script with FileReferenceList and a PHP script that moves the files and inserts to database. The file upload works well using Win XP (IE, Firefox, Opera). On the Mac (safari, Mozilla), we get IOErrors, the files won't upload. the screen just stalls. There is no authentication necessary, nor are there any restrictions for filenames etc. as the documentation suggests. We're not starting sessions, as we got errors that way, too. We tried on several machines all with latest flash player installed, it works on the PC, doesn't on the mac. We tried on two different servers, I even removed all PHP code from the upload script just to make sure it's not a server-side problem, so obviously there is something awkward happening on the Mac. If you are familiar with this and can fix this problem for us, please bid or post message to pmb.