Andreas Röhler <andreas.roehler@easy-emacs.de> writes:

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)’.

I'm unable to reproduce this in Emacs 29.

emacs -Q
M-:  (setq calendar-mark-diary-entries-flag t
       calendar-view-diary-initially-flag t)
M-x calendar

I don't get any errors.  I guess I need a diary file for today to
reproduce the problem?  What do you have?

 In ~/.emacs.d/diary exists an empty foo.org

Getting in *Messages*:

diary-list-entries: Wrong type argument: stringp, nil