We've got some more information about this issue. What's happening is that Amazon is separating the seller accounts from the associate accounts. The Pedia programs only support the associate functions of the API as they are meant to download information so that you can then use your associate ID within our programs or exported web pages.
For our users this means you need to have a valid Associate ID linked to the same account that you are using for the access key, i.e. they both have to be linked to the same email address. Please visit the associate portal
and make sure that you are registered with the same email address that you are using for the Amazon Web Service that gives you the access key. Then make sure to add your associate ID in the provided field in the Pedias -> Preferences -> Sites -> Amazon Settings panel.
If that does not solve the issue, try registering an associate account
with a new email address. Once logged in, take note of your new associate ID and follow the left side hand links that says "Product Advertisement API" and use the sign-up button. After signing up you will end up at the same security credential page that provides you your Access Key and Secret Key necessary. With that information you can then fill out the Amazon Settings panel in the Pedias again.
If you are still having issues after signing up with a new email and associate account, then follow this link
and write to Amazon stating your access key and corresponding associate ID that is registered with the same email address. There is no need to send them your secret key.
There is more information about the recent changes in the Amazon forums, for example here