Have you ever encountered a 403 error on a SharePoint site collection?

Personally, I am not able to reproduce this behavior. This being said, I already encounter this problem with one of my customers on a sites collection with data that cannot be lost.

I still haven't found the cause; however, the property of the sites collection that causes it is -LockState.

Below, the SharePoint Online PowerShell cmdlet that allows to unblock the sites collection:

Set-SPOSite https://contoso.sharepoint.com/sites/traget-site -LockState "Unlock"

Here, the PnP PowerShell cmdlet that allows to unblock the sites collection:

Set-PnPTenantSite -Url https://contoso.sharepoint.com/sites/traget-site -LockState Unlock