As suggested I logged out & logged in again. I refreshed all the pages.
The problem is still there. From a browser window I cannot add to a list or delete caches from a list.
I am using a MacBook Pro running macOS 10.15.7 and using a Safari browser 15.6.1.
I can replicate the same problem on another mac and on my iPad.
This issue is only when working with lists in a browser. Using Cachly on my iPad I can add/delete to a list through the API.