Found this to be the solution:
This might happen to you if you are using the Microsoft Internet
Explorer browser version 6.x that has a buggy Service Pack applied to it
through Windows Update. The buggy patch to IE causes the browser to
intermittently send no POST data when posting to a form that is
encrypted with SSL.
You can tell if your Internet Explorer browser has the bad update on it
by clicking Help, About Internet Explorer from the top menu and then
looking for patch number Q832894 in the patch list.
If you find that patch, which is the one that causes Internet Explorer
to be broken when communicating with secure forms, then you need to get
the next patch mentioned in this Microsoft Knowledge Base article that
patches the broken patch. The article identifier is KB831167 if you need
to search for it in the Microsoft archives.
The solution to the problem is to download the patch file named
Q831167.exe, save it to your Desktop, then doubleclick the file to run
it. Running the file will install the patch and prompt you to restart
your computer. After you reboot your computer, you should see that patch
in the list of patches when you look at About Internet Explorer.
The knowledge base article at Microsoft also offers editting your
registry as an alternative to downloading the patch. You should never
edit your registry unless you know what you are doing, and editting the
registry is not necessary if you have downloaded and installed the above
Q831167 patch.
At the time of this writing, February 26, 2004, patch Q832894 (the bad
one) is applied automatically through the Windows Update. However, patch
Q831167 (the good one) that fixes the intermittent empty post problem,
is not available through Windows Update and must be searched for
manually by Internet Explorer users.
No other browsers (Mozilla, Netscape, Firefox, Galleon, etc) experience
this empty POST problem. It only occurs with Internet Explorer 6
browsers that have Q832894 without Q831167.
The problem has nothing do do with Apache, PHP, or any cgi script and
cannot be fixed with any script programming. This problem can only be
fixed by the site visitor fixing their browser