Hi Julia,
I checked your Creative Cloud subscription and it is active, so you should not get any prompt for Serial no.
Please follow the below mentioned steps to fix the issue.
Check the host file and delete any Adobe entries from it.
Windows:
Navigate to following location:
/windows/system32/drivers/etc
1. look out for "Hosts" file
2. Open it with notepad
3. Check if you have any entry for Adobe
4. Remove the entries and try again launching any product from CS6
If you are unable to edit the Hosts file then you can run the Microsoft Fixit for the same.
http://support.microsoft.com/kb/972034
Mac:
1. Please click on "Go" and navigate to /private/etc
2. Open "hosts" file and check out for any entries for Adobe.com
3. Remove the entries and save the file
4. Launch any CC Apps and try to license it.
Please let us know if it helps.
Regards,
Abhijit