Does this only happen with large files? It is likely that the session is timing out. This is especially true if no other users happened to be logged on to the Web Services. First try increasing the "Idle time-out" value in the Advanced Settings of the Web Service Application Pool for MediaBin.
From the description, it looks like applet is loading and shows the file transfer status as 0%. If it is applet based transfers, you can also enable the Java console on Mac and see if any errors.
What's the Java version on Mac?