Hi, I am in the process of creating the interaction between menu and game mode. I can go from the menu to the game mode via a button. I have assigned the Esc key to return to the first scene with the main menu. But back in the menu the buttons are not clickable anymore. Does anyone know what the reason could be?
Check if you still have EventSystem object when you back to the menu. It seems like something is preventing you from interacting with the UI. It may be EventSystem itself or GraphicsRaycaster on the Canvas.
Hi Aldeminor, thanks for the quick response.
The EventSystem is still there when I return to the Menu. But inside the game I am using the Player and CharacterController script (Kinematic Character Controller) for inputs which I enhanced for Escape key handling. Beside that I am not using a Canvas Container since I use UI Document (UI Builder). 
Given what you said, I still can not infer the complete circumstances of the incident. To resolve the situation I suggest you to create a micro demo project that reproduces the situation. Include no assets, only the script that controls scene transition. Repeat your setup with two scenes, make sure that the setup reproduces the issue. Publish the project on Github, don’t forget to initialize the repository with gitignore file for Unity projects, then reply here and I’ll look into it.