Michael Albinus <michael.albinus@gmx.de> writes:

> Felix Dietrich <felix.dietrich@sperrhaken.name> writes:
>
>>> Anyway, when `tramp-archive-autoload-file-name-handler' is present after
>>> tramp-archive-enabled has become nil, it is used and called but doesn't
>>> handle the request (i.e. returns nil).  AFAIU, that was what I have been
>>> seeing.
>>
>> Could the tramp error handler provide a more helpful error message

Oh, I meant “tramp file handler” here instead of “tramp error handler” –
although I had made wrong assumptions; therefore, this is irrelevant.

> Since this bug is fixed now, I don't believe it is worthful to invest
> more code there.

I am *not* convinced that the reason for this bug has been correctly
identified.  After spending some more time with it, I have come up with
the following test case with which I am now able to produce the error
reliably with revision a5841b196f [1] (that is your patch):

  #+begin_src emacs-lisp
	(let* ((emacs-repo "~/src/emacs/")
		   (foo.iso (concat (file-name-as-directory emacs-repo)
							"test/lisp/net/tramp-archive-resources/foo.iso/")))
	  (load "tramp-archive")
	  (let ((file-name-handler-alist
			 (list (cons (tramp-archive-autoload-file-name-regexp)
						 #'tramp-archive-autoload-file-name-handler)))
			(tramp-archive-enabled nil))
		(file-directory-p foo.iso))))
  #+end_src

With Emacs 27.1, however, I was not able to reproduce the issue.
Therefore, I used “git bisect” between the tag “emacs-27.1” and your
commit a5841b196f with the attached script to find the offending commit
introducing the issue: 4db69b32b8 (“Fix bug#48476”) [2].