Product API always return invalid signature

I've been waiting for more than a week now (Support told me it can take up to 72h for the key to get activated), but when trying to make a request via the ScratchPad I always get:

Error! InvalidSignature The request has not been correctly signed. If you are using an AWS SDK, requests are signed for you automatically; otherwise, go to https://webservices.amazon.com/paapi5/documentation/sending-request.html#signing.

I tripple checked my keys, they are correct. I reset them before and also made sure the region is correct.

I simply want to be able to search items via the api with a EAN. I read in the docs that it is possible but before implementing it with Flutter I want to make sure that my setup is correct via the Scratchpad.

What am I missing here? What can I do?