Issues with Adobe Stock API

Hi guys,

I'm trying to use the Adobe Stock API in combination with the provided PHP SDK for Adobe Stock (GitHub) and hope that someone can help me solve the issues that I'm currently facing.  I created a new integration at the adobe console which provided me with the necessary API key and client secret for OAuth. When I'm now sending a request with Postman using the given API key everything works fine like you can see here:

However when I'm trying to do the same basic request with the Stock API PHP library I always get the response:

{"error_code":"403003","message":"Api Key is invalid"}

Since the library is using GuzzleHttp I dumped the header of the sent request in order to debug a little further and to confirm that the correct api key has been sent.

I blackened most of the api key but I'm 100% sure that it is correct.

Has someone of you guys experienced something like that? Or has someone a advice on how I should further debug this issue?

By the way: I also obtained an access token using the User Auth UI but apparently the token is not necessary for requests on /Search/Files.

Thanks for your help!

Andreas