When using 2 way authentication with Office 365 you must generate app passwords when – for example – using Skype for Business. Use the following steps to generate a (new) password :
Go to the Microsoft Office 365 portal and sign in (portal.microsoft.com)
Go to settings (Landing page, upper right)
and choose voor Instellingen voor Office 365 (Dutch)
Choose Aanvullende beveiligingsverificatie (Dutch)
Click the link to change your settings :
Go to appwachtwoorden (Dutch)
Create an App password. You can give your password a specific name (for example Skype). The password will be random generated. Use that password in Skype.