You dont have to use force.com site. You can simply use login.salesforce.com or my domain for end users
Follow these steps to makes sure site has access Configuring the Self Service site for clients - Documentation for BMC Remedyforce 20.19.02 - BMC Documentation
temporary fix (until Winter 21)
Deselect Secure guest user record access
- To access this setting, from Setup enter Sharing Settings in the Quick Find box.
- Select Sharing Settings and scroll down to the setting.
Deselect Assign new records created by the Salesforce Sites guest users to a default owner in the org
- To access this setting, from Setup enter Sites in the Quick Find box.
- Select Sites and scroll down to the setting.
Longer term solution maybe? - I'm not sure if this will still work after Winter 21
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Just had the same issue.
In setup search for "Sites", Click the site Label for the self service portal (Not Edit).
Select the "Public Access Settings" button
Select Object settings
On both "SelfService Images" and "SelfService Themes" give everyone read and "View All" Access