GET User 404 - Even with correct login name

Trying to get the user ID for a user / channel name - but it returns 404.
The login name used there is also correct and exists (also tried with other channels, but same result: 404)

The URL I try to call:

Anyone knows what is wrong?

Can you show us the code you’re using to make the request, because that endpoint works perfectly fine for me so it’s likely something in your code is making an erroneous request.