Re: After using CS6 for a year, I suddenly get a message that my serial is invalid.

I assume you have Master collection CS 6 installed on your machine as we had a separate installer for Master collection suite which has all the CS 6 apps. if so, Please delete the SLStore folder from location below and give a try again.

C:\ProgramData\Adobe\SLStore

Please note serial key for Master collection CS 6 will not work with any creative cloud app.

Thanks,

Ashish