Create a "App Password"

If you have 2FA (two factor authentification) enabled, there is no way apps like calendar, contacts (trough CalDAV, CardDAV protocols)  or other internet protocols such as WebDAV can sync with your account. To enable this feature, Nextcloud lets you create "App Passwords" trough your account settings. Even if you change your Account's Password, the "App Passwords" so you do not need to re-connect all your clients and apps.

To create a new "App Password", go into your account's security settings: