Hi! Thanks for replying. Your reply got me troubleshooting. And I got it working. But I think I may have still found a bug.
I am using the Astra Pro theme with Gutenberg btw. I tried 3 things.
(1) I inserted the form on the page via the block editor. That gave me the form fields to configure the form including adding the list name. That produced the error above.
(2) Then after reading your reply I tried with short code [newsletter_form list=listname]. This produced the same error.
(3) Then I tried with just the short code [newsletter_form]. No error and list automatically assigned to a new subscriber (my list is enforced).
I tested it further by defining another list. I then added this second list specifying the list name with short-code. The reported admin notification then appeared again.
So, all fixed for now and fixed for my needs but still a bug found.