Any error message when external users open the shared site collection?
Please check following tips:
1.Enable external sharing both in the site and tenant level.
(1)Go to SharePoint admin center -> Polices -> Sharing -> Allow Anyone or New and existing guests.
(2)Go to SharePoint admin center -> Polices -> Sharing -> Allow Anyone or New and existing guests.
2.Go to the Site settings -> Site collection features -> Deactivate "Limited-access user permission lockdown mode" feature.
3.Go to Microsoft Entra ID at https://entra.microsoft.com/ -> In the left navigation pane, expand External identities -> External collaboration settings -> Make sure that "Anyone in the organization can invite guest users including guests and non-admins" is selected.
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