Welcome to the forum.
This might be a case of needing to call Adobe Customer Support.
The "subscription" version is quite new, and probably most here have never used it. Also, with S/N issues, the best that the majority here can offer is based on reading of problems that others have posted of.
Hope that someone here has a fix for your issue, but I'd be planning on making that call to C/S (not Technical Support), and having all documentations, invoices, etc. handy.
I've downloaded and installed Photoshop CS5.1
Where did you download it from?
Are you installing the
and entering a code for the normal Photoshop product by any chance?