The issue you are facing:
I have a calendar I created in NextCloud added to Thunderbird (TB), using no addons, just the native support.
Now I want to add a second synced calendar. I created it in the NextCloud web UI, copied the URL, but when I try to add it to TB, I need to give a password.
But really the existing app password should suffice to access this new calendar. I just can’t access it neither in TB nor in the NextCloud web UI, and TB doesn’t reuse it automatically. Is there a way to avoid generating a second app password for the same app?