Came across "incomprehensible buffer" error in package.el. Workaround patch:
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -1161,6 +1161,7 @@ package--with-work-buffer
(let* ((url (concat ,url-1 ,file))
(callback (lambda (status)
(let ((b (current-buffer)))
+ (goto-char (point-min))
(unwind-protect (wrap-errors
(when-let ((er (plist-get
status :error)))
(error "Error retrieving: %s %S" url er))