KB486467: “Authentication Error. Your login has been denied by the system." error shows while attempting to log in MicroStrategy Library using SAML authentication in MicroStrategy ONE Update 12

Symptom

While trying to log in, users consistently receive “Authentication Error. Your login has been denied by the system." error message.
 

Following entries can be found in the SAML.log:
 

Invalid assertion [id112341234123412345] for SAML response [id112341234123412345]: 
Condition '{urn:oasis:names:tc:SAML:2.0:assertion} AudienceRestriction' of type 'null' 
in assertion 'id112341234123412345' was not valid.:  
None of the audiences within Assertion 'id112341234123412345' matched the list of valid audiances (…)

<saml2:AudienceRestriction><saml2:Audience>
MstrTSSaml
</saml2:Audience></saml2:AudienceRestriction> 

  <entityId>MSTRTSSAML</entityId>

<ns0:EntityDescriptor xmlns:ns0="urn:oasis:names:tc:SAML:2.0:metadata" xmlns:ns1=
"http://www.w3.org/2000/09/xmldsig#" ID="MSTRTSSAML" entityID="MSTRTSSAML">

Solution

Modifying the entityID field on the Identity Provider or Strategy side so they fully match, taking case sensitivity into account.