I have encountered no problems with the system that was upgraded from XP.
Both have recently been upgraded one from XP and one from Vista both to Windows 7.
If I choose to re-enter the full 24 digit serial number, I get a green tick telling me that this is a valid number but when activation starts I am again told that the 20 digit number is incorrect. It shows only a 20 digit number as being incorrect. As soon as I opened Acrobat, a message pops up telling me that the serial number is incorrect. I entered the serial number during installation which was accepted. I Uninstalled the software, did the clean up and reinstalled Acrobat 9. My solution of removinng the file in FlexNet only worked for one day and when the computer was resarted the following day, the same problem occured.
