When trying to open an Office document stored on an Extradium-protected web application, the user cannot sign in within Office neither with an Extradium user account nor with a Windows user account.
This issue is caused by SharePoint 2016 trying to enforce a "modern authentication" scheme (OAuth), which Forms-based Authentication is not compliant with.
- On your SharePoint 2016 server, open a SharePoint 2016 Management Shell (with Administrator privileges) and type the following commands:
$sts = Get-SPSecurityTokenServiceConfig
$sts.SuppressModernAuthForOfficeClients = $true
- Next, restart IIS and the SharePoint Timer Service with the following commands:
net stop SPTimerV4
net start SPTimerV4
The resolution is explained in more detailed on this official Microsoft support page.