3367601 - SAML configuration error.User [xxx] does not have an attribute [yyy] or fallback attribute [none] - Identity Authentication Service

When trying to log into your application, you get this error Identity Provider could not process the authentication request received. Delete your browser cache and stored cookies, and restart your browser. If you still experience issues after doing this, please contact your administrator. 

If you check the logs, you see the following:
SAML configuration error.User [null] does not have an attribute [${customAttribute1}] or fallback attribute [none] set Caused by: Name id attribute ${customAttribute1} must match at least one valid and supported user attribute value. 

or

SAML configuration error.User [xxx] does not have an attribute [loginName] or fallback attribute [none] set Caused by: Name id attribute ${customAttribute1} must match at least one valid and supported user attribute value.

Identity Authentication

Identity Provider could not process the authentication request received. Delete your browser cache and stored cookies, and restart your browser. If you still experience issues after doing this, please contact your administrator , KBA , BC-IAM-IDS , Identity Authentication Service , LOD-SF-PLT-IAS , Identity Authentication Services (IAS) With BizX , Problem