Hi @Beran Karel ,
1.Please check you Sharing and Active Sites setting again. Make sure you have sharing settings enabled properly.
2.This issue may also occur if the site collection feature Limited-access user permission lockdown mode is activated. You could try to deactivate Limited-access user permission lockdown mode feature.
Settings > Site Information > View all site settings > Site collection features > Limited-access user permission lockdown
3.You could try to use PowerShell code to set SharingCapability.
PowerShell code:
Set-SPOSite -Identity your site url -SharingCapability ExternalUserAndGuestSharing
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