The DisableAudienceRestrictionCheck disables the check of the audience restriction condition against the configured local provider name.
The exception you're seeing has occurred before this check. We check the received SAML message against the SAML XML schema and this check is failing.
According to the SAML XML schema, the Audience value should be a URI. The actual value "http://srv-stg-app1.infra-nprd.com:Legacy"
isn't a URI.
the value you supplied to the IdP as the provider name (aka entity ID)?
If so, I recommend changing this to a legal URL (eg "http://srv-stg-app1.infra-nprd.com/Legacy")
and providing this to the IdP and updating your LocalServiceProviderConfiguration.Name.
If this isn't possible, you'll have to disable the XML schema check by setting the LocalServiceProviderConfiguration.DisableSchemaCheck to true.
"Description": "Example Service Provider",