I had the same problem.. Somehow the PC ID was not the same as my installed version of Mach4 so I deleted the Mach4 install, saving my config 1st, then downloaded a fresh copy from the FTP site where the newest versions live, and grabbed the current PC ID. I deactivated the existing license and added the new PC ID and sent the license. It installed as before but this time it saved the license...
Hope this helps...
Cheers,
Hugh