for the first time i started to test the SL4P Matter <> Alexa on Android (i.e. Echo Show 8 Gen. 3, 2023 in the lets say Backend) Integration but after scanning the QR Code I always get the message, that it doesnt work and the question if I had paired it with another Assistant or App which I dont have. Or does the Alexa Skill (which I am also using for my SL3P) is also counting?
Is there something special to do or what I can troubleshoot?
Matter pairing works in several steps. First one is onboarding to the network (which obviously worked, because your lock is connected to Thread) and afterwards integration into a fabric and then maybe some additional steps inside each ecosystem. In your case it looks a bit like the problem is inside Alexa and not really something that the Smart Lock sees (i.e. pairing was succesfully completed from Smart Lock side).
In this case you’ve to contact Amazon support with the error code provided on the screenshot.