Salesforce are due to retire their default certificate on August 7th 2017 details of this can be found here. If your organisation uses Single Sign On and you are using the default certificate this service will stop working on this date.
Sage People have produced a guide to enable you to check if this affects your organisation: https://www.fairsail.com/sage-people/wp-content/uploads/2017/05/How-to-check-for-default-certificate.pdf
Fyi Sage People have also produced a guide to replace this certificate when using ADFS here:
If you use Okta, Azure or Onelogin as your authentication service, please note these guides are currently being created, so watch this space.
If you need help replacing certificates please call Tugela on +44 (0)1327 317701