Encountered a situation recently where I had to reenter my product key whenever I opened a Microsoft Office 2011 application, even though my software was already licensed.
Microsoft’s support site states that the error could be triggered because the licensing file is either missing or corrupted or because the Installation ID for the license does not match the installation ID of the computer on which Office for Mac is installed.
Microsoft offers several methods for resolving this type of error. One suggestion was to move three licensing files from the Mac’s Library folder to the trash:
I was required to enter my admin password to trash the first two files but not the third. After I moved the files, Office 2011 would not start nor did the program ask me to enter my product key again.
Undaunted, I decided to see what would happen if I moved those same three licensing files back to their original locations (hint: Don’t empty the trash!), reentering my admin password in order to complete the process.
Voilà! Success! Office 2011 started up in a normal fashion without asking for the software product key. It is now apparent that the problem was triggered by a file permissions conflict, and by transferring the licensing files back-and-forth on my Mac, I restored permissions to their proper setting.