I am having the same issue. My DeveloperID is reporting as “invalid” when a seller puts it in their account. Also I get “access denied” when I use my own developer credentials against my own account.
I opened ticket 5785855061 with support, and the first reply said:
“Upon checking with my internal tools, I see that the MWS authorization for your account is deactivated which is why you receiving the error while trying to access API using your MWS keys”
I received no notice that this deactivation took place. There is no indication in my seller account that my developer id is not active. My developer ID worked previously, but I hadn’t used my developer ID for the last few months because of focus on other projects. I understood Amazon was cracking down on giving out new Developer IDs.
I am aware of one DeveloperID that was registered some years ago, has been in active daily use, and its still active, without having to go through the new developer registration process.
I am aware of two other Developer IDs (other than my own) who applied under the old procedure, but haven’t been actively used recently, and those Developer IDs also deactivated.
My current assumption is Amazon deactivated developer IDs that haven’t been used for a period of time, and is making all of those users reapply from scratch.
The first reply from tech support also said:
“Hence, to get the MWS ID and ACCESS key, you have to register again as a Developer. You can apply for Amazon MWS Developer API Access keys from your Seller Central account, from Settings > User Permissions, and clicking the “Visit Developer Credentials” button.” … “Once you apply, Amazon will thoroughly review your business information and intended use case before granting MWS access keys. This process may take up to 30 days.”
The problem is, once on the “Developer Central” page, THERE IS NO BUTTON OR LINK TO REAPPLY to be a developer. (The page only shows my existing, and deactivated, Developer ID.)
In my first reply to the ticket, I documented this fact including providing a screen clip of my “Developer Central” screen. In addition, I asked when and why my existing Developer ID has been deactivated.
In the second reply I got from tech support, my issue of not finding the link to re-apply, in addition to my questions about my previously active developer ID, WERE IGNORED. The only thing I got back in the second reply from tech support was a number of links to the instructions I was already provided in the first reply. (The exact same email as BrickOvenMaker received as noted above my post.)
I just submitted again the issue of no apply link showing up on my “Developer Central” page, and I attached the same screen clip as in my first reply.
It seems my ticket was routed to a different department, as I got the second reply from "firstname.lastname@example.org". So it seems the ticket has been routed to “developer registration vetting”, so I guess that is some progress.
I will keep this thread updated.