GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 2.24.32, 
cairo version 1.16.0) of 2022-04-06

Error from M-x calendar RET, backtrace below.

Caused by custom-file content of

  '(calendar-mark-diary-entries-flag t)
  '(calendar-view-diary-initially-flag t)

Error from ‘diary-fancy-display’

(with-current-buffer (find-buffer-visiting diary-file)

and get ‘nil’ from ‘(find-buffer-visiting diary-file)’.

Thanks,

Andreas

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
   diary-fancy-display()
   diary-list-entries((4 12 2022) nil)
   diary-view-entries()
   calendar-basic-setup(nil)
   calendar(nil)
   funcall-interactively(calendar nil)
   call-interactively(calendar record nil)
   (prog1 (call-interactively cmd record-flag keys) (if (and (symbolp 
cmd) (get cmd 'byte-obsolete-info) (not (get cmd 
'command-execute-obsolete-warned))) (progn (put cmd 
'command-execute-obsolete-warned t) (message "%s" 
(macroexp--obsolete-warning cmd (get cmd 'byte-obsolete-info) "command")))))
   (cond ((arrayp final) (if record-flag (progn (add-to-history 
'command-history (list 'execute-kbd-macro final prefixarg) nil t))) 
(execute-kbd-macro final prefixarg)) (t (prog1 (call-interactively cmd 
record-flag keys) (if (and (symbolp cmd) (get cmd 'byte-obsolete-info) 
(not (get cmd 'command-execute-obsolete-warned))) (progn (put cmd 
'command-execute-obsolete-warned t) (message "%s" 
(macroexp--obsolete-warning cmd (get cmd ...) "command")))))))
   (let ((final cmd)) (while (progn (setq final (indirect-function 
final)) (if (autoloadp final) (setq final (autoload-do-load final 
cmd))))) (cond ((arrayp final) (if record-flag (progn (add-to-history 
'command-history (list 'execute-kbd-macro final prefixarg) nil t))) 
(execute-kbd-macro final prefixarg)) (t (prog1 (call-interactively cmd 
record-flag keys) (if (and (symbolp cmd) (get cmd 'byte-obsolete-info) 
(not (get cmd ...))) (progn (put cmd 'command-execute-obsolete-warned t) 
(message "%s" (macroexp--obsolete-warning cmd ... "command"))))))))
   (if (and (symbolp cmd) (get cmd 'disabled) (or (and (setq query (and 
(consp (get cmd ...)) (eq (car ...) 'query))) (not 
(command-execute--query cmd))) (and (not query) 
disabled-command-function))) (if (not query) (progn (run-hooks 
'disabled-command-function))) (let ((final cmd)) (while (progn (setq 
final (indirect-function final)) (if (autoloadp final) (setq final 
(autoload-do-load final cmd))))) (cond ((arrayp final) (if record-flag 
(progn (add-to-history 'command-history (list ... final prefixarg) nil 
t))) (execute-kbd-macro final prefixarg)) (t (prog1 (call-interactively 
cmd record-flag keys) (if (and (symbolp cmd) (get cmd ...) (not ...)) 
(progn (put cmd ... t) (message "%s" ...))))))))
   (let ((prefixarg (if special nil (prog1 prefix-arg (setq 
current-prefix-arg prefix-arg) (setq prefix-arg nil) (if 
current-prefix-arg (progn (prefix-command-update)))))) query) (if (and 
(symbolp cmd) (get cmd 'disabled) (or (and (setq query (and (consp ...) 
(eq ... ...))) (not (command-execute--query cmd))) (and (not query) 
disabled-command-function))) (if (not query) (progn (run-hooks 
'disabled-command-function))) (let ((final cmd)) (while (progn (setq 
final (indirect-function final)) (if (autoloadp final) (setq final 
(autoload-do-load final cmd))))) (cond ((arrayp final) (if record-flag 
(progn (add-to-history ... ... nil t))) (execute-kbd-macro final 
prefixarg)) (t (prog1 (call-interactively cmd record-flag keys) (if (and 
... ... ...) (progn ... ...))))))))
   command-execute(calendar record)
   (let ((prefix-arg prefixarg)) (command-execute function 'record))
   (let* (#'(and (stringp command-name) (intern-soft command-name)) 
(binding (and suggest-key-bindings (not executing-kbd-macro) 
(where-is-internal function overriding-local-map t))) 
(delay-before-suggest 0) (find-shorter nil)) (if (commandp function) nil 
(error "`%s' is not a valid command name" command-name)) 
(set--this-command-keys (concat "\370" (symbol-name function) "\15")) 
(setq this-command function) (setq real-this-command function) (let 
((prefix-arg prefixarg)) (command-execute function 'record)) (if 
execute-extended-command--binding-timer (progn (cancel-timer 
execute-extended-command--binding-timer))) (if (and suggest-key-bindings 
(or binding (and extended-command-suggest-shorter typed))) (progn (setq 
delay-before-suggest (cond ((= 0 (length ...)) 0) ((numberp 
suggest-key-bindings) suggest-key-bindings) (t 2))) (if (and 
extended-command-suggest-shorter (not binding) (not executing-kbd-macro) 
(symbolp function) (> (length (symbol-name function)) 2)) (progn (setq 
find-shorter t))) (if (or binding find-shorter) (progn (setq 
execute-extended-command--binding-timer (run-at-time 
delay-before-suggest nil #'...)))))))
   execute-extended-command(nil "calendar" nil)
   funcall-interactively(execute-extended-command nil "calendar" nil)
   call-interactively(execute-extended-command nil nil)
   (prog1 (call-interactively cmd record-flag keys) (if (and (symbolp 
cmd) (get cmd 'byte-obsolete-info) (not (get cmd 
'command-execute-obsolete-warned))) (progn (put cmd 
'command-execute-obsolete-warned t) (message "%s" 
(macroexp--obsolete-warning cmd (get cmd 'byte-obsolete-info) "command")))))
   (cond ((arrayp final) (if record-flag (progn (add-to-history 
'command-history (list 'execute-kbd-macro final prefixarg) nil t))) 
(execute-kbd-macro final prefixarg)) (t (prog1 (call-interactively cmd 
record-flag keys) (if (and (symbolp cmd) (get cmd 'byte-obsolete-info) 
(not (get cmd 'command-execute-obsolete-warned))) (progn (put cmd 
'command-execute-obsolete-warned t) (message "%s" 
(macroexp--obsolete-warning cmd (get cmd ...) "command")))))))
   (let ((final cmd)) (while (progn (setq final (indirect-function 
final)) (if (autoloadp final) (setq final (autoload-do-load final 
cmd))))) (cond ((arrayp final) (if record-flag (progn (add-to-history 
'command-history (list 'execute-kbd-macro final prefixarg) nil t))) 
(execute-kbd-macro final prefixarg)) (t (prog1 (call-interactively cmd 
record-flag keys) (if (and (symbolp cmd) (get cmd 'byte-obsolete-info) 
(not (get cmd ...))) (progn (put cmd 'command-execute-obsolete-warned t) 
(message "%s" (macroexp--obsolete-warning cmd ... "command"))))))))
   (if (and (symbolp cmd) (get cmd 'disabled) (or (and (setq query (and 
(consp (get cmd ...)) (eq (car ...) 'query))) (not 
(command-execute--query cmd))) (and (not query) 
disabled-command-function))) (if (not query) (progn (run-hooks 
'disabled-command-function))) (let ((final cmd)) (while (progn (setq 
final (indirect-function final)) (if (autoloadp final) (setq final 
(autoload-do-load final cmd))))) (cond ((arrayp final) (if record-flag 
(progn (add-to-history 'command-history (list ... final prefixarg) nil 
t))) (execute-kbd-macro final prefixarg)) (t (prog1 (call-interactively 
cmd record-flag keys) (if (and (symbolp cmd) (get cmd ...) (not ...)) 
(progn (put cmd ... t) (message "%s" ...))))))))
   (let ((prefixarg (if special nil (prog1 prefix-arg (setq 
current-prefix-arg prefix-arg) (setq prefix-arg nil) (if 
current-prefix-arg (progn (prefix-command-update)))))) query) (if (and 
(symbolp cmd) (get cmd 'disabled) (or (and (setq query (and (consp ...) 
(eq ... ...))) (not (command-execute--query cmd))) (and (not query) 
disabled-command-function))) (if (not query) (progn (run-hooks 
'disabled-command-function))) (let ((final cmd)) (while (progn (setq 
final (indirect-function final)) (if (autoloadp final) (setq final 
(autoload-do-load final cmd))))) (cond ((arrayp final) (if record-flag 
(progn (add-to-history ... ... nil t))) (execute-kbd-macro final 
prefixarg)) (t (prog1 (call-interactively cmd record-flag keys) (if (and 
... ... ...) (progn ... ...))))))))
   command-execute(execute-extended-command)