I have just installed and configured ArcGIS Notebook Server 10.8.1. Notebook was federated with ArcGIS Portal, I can successfully access Notebook Server Manager where everything looks great, and I got a new tab in my Portal - 'Notebook'. However, when I try to open it, I get the notebook interface, but the notebook itself cannot be loaded. I get an error 499 stating that token is required.

Inside developer tools:

Which step of configuration am I missing?
I will appreciate any help!