I've been using Creative Suite 5.5 Design Premium since 2014. I opened it the other day and an Adobe Application Manager dialog box came up with a prompt to provide serial number, however, when I enter it it says "This serial number is not valid for this product". It gives me no other choice but to continue as a trial for 30 days. I'm very upset as I paid over $700 for this program. Can anyone help me with this. I'm on a Mac.
With problems on you serial, no one can help you in the forum. You have to contact Adobe directly.
As you did not mention, which version of the OS you are using, you have to be aware, that CS5.x is not supported with El Capitan or Sierra.
If you registered your product, the serial number will be listed in your account at Adobe.com.
Though the price is seriously, alarmingly low unless it was an upgrade. From whom was it purchased?