If you get mastodon-auth--handle-token-response: Mastodon-auth--access-token: invalid_grant: The provided authorization grant is invalid, expired, revoked, does not match the redirection URI used in the authorization request, or was issued to another client. while trying to set up mastodon.el from MELPA, you might have an outdated version. Try the following steps:

• M-x package-delete mastodon
• Restart Emacs
• M-x package-refresh-contents
• M-x package-install mastodon

If that doesn't work, try installing it from the Git repository: https://codeberg.org/martianh/mastodon.el.git

Posting this here to make it searchable.