Problem with CS5.1

[Matthijs329626408zn3]

 I have been using Photoshop CS5.1 for over 10 years. Now I suddenly have to enter a serial number or 
choose the option to continue using the "trial version" for 30 days. How is this possible? I work off-line. 
More importantly: how do I solve this? Can anyone help me with this?

[kglad]

enter the serial number.

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in the future, to find the best place to post your message, use the list here, https://community.adobe.com/

p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.

<"moved from using the community">

[Peru Bob]

Do as suggested by kglad .

Did you make any recent major hardware changes (internal hard drive, mobo, processor, etc}?

To find your serial number:

https://helpx.adobe.com/download-install/kb/find-serial-number.html

[Matthijs329626408zn3]

Thank you both for the quick response. Sorry, I wasn't clear. Although I wondered why I need to enter a 
serial number when I have been working with this version of Photoshop for over 10 years, I entered the 
serial number. But then I get to a screen where I have to enter my email address. However, a few years 
ago I switched email providers. That's why I'm stuck in this screen. How could I solve this?

[Ben M]

Thank you both for the quick response. Sorry, I wasn't clear. Although I wondered why I need to enter a serial number when I have been working with this version of Photoshop for over 10 years, I entered the serial number. But then I get to a screen where I have to enter my email address. However, a few years ago I switched email providers. That's why I'm stuck in this screen. How could I solve this?

By @Matthijs329626408zn3

Switching email providers would have no impact on this at all. You say you are stuck on the screen to enter a serial number? I'm pretty certain the CS5 servers are offline. You may be able to try an offline activation: https://exception.licenses.adobe.com/aoes/aoes/v1/t1?locale=en . 

[kglad]

disconnect your computer from the internet.

start your cs5 app

carefully follow #2 here https://helpx.adobe.com/creative-suite/kb/serial-number-validation-launching-cs6.html

[Jeffrey_A_Wright]

Matthijs, I am sorry you cannot enter the serial number for Photoshop. You may be asked to reenter the serial number for various reasons. Peru Bob's advice is valid about changes in hardware, or even security software, that could cause the registration and activation to be removed from the computer.

Please try the suggestions by Ben M and Kglad to see if you can complete the offline activation process.

If you continue to encounter problems, I recommend verifying you have access to the account with Photoshop is registered under. You mentioned you changed e-mail providers, Mathijs. You can use the steps listed in https://helpx.adobe.com/manage-account/using/change-email-address.html to change the e-mail address assigned to the account. 

Alternatively, you may need to complete a transfer of license process to transfer Photoshop to the account you used to post to this public discussion forum. Information on how to request a transfer can be found at https://helpx.adobe.com/x-productkb/policy-pricing/transfer-product-license.html