Having the same issue but with hr.applicant. Any solutions ?

In Settings, SUBMIT sitemap to Google, shows valid sitemap URL (https://domain.com/sitemap.xml. 
When I visit this URL on RUNBOT test instance, all works OK (It shows SITEMAP in XML format) , 
BUT
when I visit the URL on my domain, I get ERROR 403

403: Forbidden

The page you were looking for could not be authorized.

A mistake

Error report:

You are not allowed to access 'Appointment Type' (appointment.type) records.

This operation is allowed for the following groups:
	- Types of users/Internal user
	- Online meeting/Admin

If necessary, ask an administrator for access.

QWeb

Error report:

Error while rendering the template
AccessError: You are not allowed to access 'Appointment Type' (appointment.type) records.

This operation is allowed for the following groups:
	- Types of users/Internal user
	- Online meeting/Admin

If necessary, ask an administrator for access.
Template: website.sitemap_locs
Path: /t/url
Node: 

The error occurred while rendering the template website.sitemap_locsand evaluating the following expression:

Track down

If you want anonymous users to be able to access this page, you'll need to add the permissions to this resource to the User Types/Public group. 

That way anyone can access the page without logging in. If you don't want this page to be accessible then you can either add this URL to the robots.txt file so it won't be crawled and won't generate an error, I'm not sure if there's a way to prevent it from being included in the sitemap. If you don't need this appointment type you could always delete it which will remove it from the sitemap.

I'm having the same issue with my website, I tried to use the sitemap of Odoo attachments (domain.com/sitemap-1.xml) but it didn't work, and it is also bad generated. How can we fix it?