Scott Otterson <scotto@sharpleaf.org> writes:

> Having just updated to emacs 28.1, I'm seeing the warning
>
> org-element--cache: Unregistered buffer
>
> every time I open a .org file.  I also notice that org-superstar no longer
> works.
>
> These problems persist if I delete my .emacs.d and let straight.el rebuild
> everything.

Thanks for reporting! You appear to use straight.el. Can you please
share the output of M-x org-version? straight.el is known to create
problems when Org is loaded late in you config.
See https://github.com/radian-software/straight.el/issues/947

Also, can you try to open the problematic files using bare Emacs
starting from emacs -Q? See https://orgmode.org/manual/Feedback.html

Best,
Ihor