This happens when the store password is changed and you are using the Complete Store Protection User feature. To make SSO work again :
- Click on lock-shaped icon in the address bar of the browser and than click on cookies.
- After that click on tab with name similar to your store domain and than click on submenu – Cookies.
- Now search for storefront_digest variable and then click on it. After that you can see it’s value under the content section as shown below. Copy this value.
- Paste the storefront_digest cookie value in the store access cookie section as shown below and then click on the Save button.
SSO will work fine again.