Amazon Prodcut API Invalid Signature

I want to use the Amazon Product API in my Flutter project.

I am testing my credentials via the Scratchpad but 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.de/paapi5/documentation/sending-request.html#signing.

I checked everything, reset my credentials, waited 24h, 48h, 2 weeks. But still, invalid signature...

I've contacted Amazon via chat, posted in the developers forum, but no answers.

What am I missing here? Why is it not working on the Scratchpad? Is it working for anyone? Is it maybe a Scratchpad-issue?

I want to test it first, before I actually do all the implementation.

Happy for EVERY help! Let me know if you need any more information.

Solution

... how to waste 2 weeks.

So if anyone comes across this issue, make sure that you don't just copy the secret key by double clicking on the key and copy it via cmd + c, since this WILL NOT COPY THE WHOLE STRING in some cases (e.g. if there is a "/" in the String).