In a prior version, when I clicked on a SCHEDULED: <...> link, it would
popup the org-agenda view. Now (after a M-x package-list-packages U x)
it does nothing and displays No link found. Is there a way to restore
the old behaviour?

Sorry - I don't know what the version was that had this feature but probably no more than 6 months old, more likely less than 3 months old.

Emacs  : GNU Emacs 26.1 (build 1, x86_64-redhat-linux-gnu, GTK+ Version 3.23.2)
 of 2018-08-13
Package: Org mode version 9.2 (9.2-elpaplus @ /home/bhepple/.emacs.d/elpa/org-plus-contrib-20181230/)

current state:
==============
(setq
 org-src-mode-hook '(org-src-babel-configure-edit-buffer org-src-mode-configure-edit-buffer)
 org-after-todo-state-change-hook '(org-checklist org-clock-out-if-current)
 org-metadown-hook '(org-babel-pop-to-session-maybe)
 org-clock-out-hook '(org-clock-remove-empty-clock-drawer)
 org-html-format-inlinetask-function 'org-html-format-inlinetask-default-function
 org-odt-format-headline-function 'org-odt-format-headline-default-function
 org-agenda-files '("~/Sync/tamborine.org" "~/Sync/test.org")
 org-ascii-format-inlinetask-function 'org-ascii-format-inlinetask-default
 org-reveal-start-hook '(org-decrypt-entry)
 org-mode-hook '((closure (t) nil (visual-line-mode 1) (set-visual-wrap-column 0))
                 #[0 "\300\301\302\303\304$\207" [add-hook change-major-mode-hook org-show-all append local] 5]
                 #[0 "\300\301\302\303\304$\207"
                   [add-hook change-major-mode-hook org-babel-show-result-all append local] 5]
                 org-babel-result-hide-spec org-babel-hide-all-hashes
                 #[0 "\301\211 \207" [imenu-create-index-function org-imenu-get-tree] 2] org-eldoc-load)
 org-odt-format-drawer-function #[514 "\207" [] 3 "\n\n(fn NAME CONTENTS)"]
 org-archive-hook '(org-attach-archive-delete-maybe)
 org-confirm-elisp-link-function 'yes-or-no-p
 org-agenda-before-write-hook '(org-agenda-add-entry-text)
 org-metaup-hook '(org-babel-load-in-session-maybe)
 org-bibtex-headline-format-function #[257 "\300 \236A\207" [:title] 3 "\n\n(fn ENTRY)"]
 org-latex-format-drawer-function #[514 "\207" [] 3 "\n\n(fn _ CONTENTS)"]
 org-babel-pre-tangle-hook '(save-buffer)
 org-tab-first-hook '(org-babel-hide-result-toggle-maybe org-babel-header-arg-expand)
 org-checklist-export-function 'org-export-as-ascii
 org-ascii-format-drawer-function #[771 " \207" [] 4 "\n\n(fn NAME CONTENTS WIDTH)"]
 org-src-lang-modes '(("arduino" . arduino) ("redis" . redis) ("php" . php) ("C" . c) ("C++" . c++)
                      ("asymptote" . asy) ("bash" . sh) ("beamer" . latex) ("calc" . fundamental) ("cpp" . c++)
                      ("ditaa" . artist) ("dot" . fundamental) ("elisp" . emacs-lisp) ("ocaml" . tuareg)
                      ("screen" . shell-script) ("shell" . sh) ("sqlite" . sql))
 org-occur-hook '(org-first-headline-recenter)
 outline-minor-mode-hook '((lambda nil (define-key outline-minor-mode-map (kbd "C-<tab>") (quote outline-cycle))
                            (define-key outline-minor-mode-map (kbd "S-<iso-lefttab>")
                             (quote
                              (lambda nil (interactive)
                               (save-excursion (goto-char (point-min)) (outline-show-all) (outline-hide-leaves)))
                              )
                             )
                            )
                           (lambda nil (local-set-key " " outline-mode-prefix-map)))
 org-cycle-hook '(org-cycle-hide-archived-subtrees org-cycle-show-empty-lines
                  org-optimize-window-after-visibility-change)
 org-speed-command-hook '(org-speed-command-activate org-babel-speed-command-activate)
 org-replace-disputed-keys t
 org-odt-format-inlinetask-function 'org-odt-format-inlinetask-default-function
 org-font-lock-set-keywords-hook '(org-drill-add-cloze-fontification)
 org-confirm-shell-link-function 'yes-or-no-p
 org-link-parameters '(("id" :follow org-id-open) ("message" :follow org-mac-message-open)
                       ("x-devonthink-item" :follow org-devonthink-item-open)
                       ("mac-evernote" :follow org-mac-evernote-note-open)
                       ("mac-outlook" :follow org-mac-outlook-message-open)
                       ("acrobat" :follow org-mac-acrobat-open) ("skim" :follow org-mac-skim-open)
                       ("addressbook" :follow org-mac-addressbook-item-open)
                       ("x-together-item" :follow org-mac-together-item-open)
                       ("mairix" :follow org-mairix-open :store org-mairix-store-gnus-link)
                       ("eww" :follow eww :store org-eww-store-link)
                       ("rmail" :follow org-rmail-open :store org-rmail-store-link)
                       ("mhe" :follow org-mhe-open :store org-mhe-store-link)
                       ("irc" :follow org-irc-visit :store org-irc-store-link :export org-irc-export)
                       ("info" :follow org-info-open :export org-info-export :store org-info-store-link)
                       ("gnus" :follow org-gnus-open :store org-gnus-store-link)
                       ("docview" :follow org-docview-open :export org-docview-export :store
                        org-docview-store-link)
                       ("bibtex" :follow org-bibtex-open :store org-bibtex-store-link)
                       ("bbdb" :follow org-bbdb-open :export org-bbdb-export :complete org-bbdb-complete-link
                        :store org-bbdb-store-link)
                       ("w3m" :store org-w3m-store-link) ("file+sys") ("file+emacs")
                       ("doi" :follow org--open-doi-link) ("elisp" :follow org--open-elisp-link)
                       ("file" :complete org-file-complete-link)
                       ("ftp" :follow (lambda (path) (browse-url (concat "ftp:" path))))
                       ("help" :follow org--open-help-link)
                       ("http" :follow (lambda (path) (browse-url (concat "http:" path))))
                       ("https" :follow (lambda (path) (browse-url (concat "https:" path))))
                       ("mailto" :follow (lambda (path) (browse-url (concat "mailto:" path))))
                       ("news" :follow (lambda (path) (browse-url (concat "news:" path))))
                       ("shell" :follow org--open-shell-link))
 org-latex-format-headline-function 'org-latex-format-headline-default-function
 org-latex-format-inlinetask-function 'org-latex-format-inlinetask-default-function
 org-html-format-drawer-function #[514 "\207" [] 3 "\n\n(fn NAME CONTENTS)"]
 org-html-format-headline-function 'org-html-format-headline-default-function
 org-mairix-display-hook 'org-mairix-gnus-display-results
 org-mairix-gnus-select-display-group-function 'org-mairix-gnus-select-display-group-function-gg
 )