Trouble Converting IdP to Duo Universal Prompt

[Bryan K. Walton]

We having a working Shibboleth IdP (version 4.2.1) that has been 
using the older Duo iframe MFA method for the last two years or so.  
We are trying to convert it to the newer Universal prompt and have 
been following the instructions found here:

https://shibboleth.atlassian.net/wiki/spaces/IDPPLUGINS/pages/1374027959/DuoOIDCAuthnConfiguration

As far as I can tell, we have everything configured correctly, yet we
are getting an error upon the redirect to the Duo 2FA screen.

In the idp-process.log, I see the following:

--- BEGIN LOG SNIPPET ---

2022-05-04 15:38:34,948 - ERROR [org.springframework.webflow.execution.FlowExecutionException:91] -
org.springframework.webflow.execution.FlowExecutionException: Exception thrown in state 'CallSubflow' of flow 'authn/MFA'
        at org.springframework.webflow.engine.impl.FlowExecutionImpl.wrap(FlowExecutionImpl.java:573)
Caused by: org.springframework.beans.factory.xml.XmlBeanDefinitionStoreException: Line 1 in XML document from net.shibboleth.ext.spring.resource.ConditionalResource at 7c77cdee is invalid; nested exception is org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Premature end of file.
        at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.doLoadBeanDefinitions(XmlBeanDefinitionReader.java:402)
Caused by: org.xml.sax.SAXParseException: Premature end of file.
        at java.xml/com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:204)

--- END LOG SNIPPET ---

In the user's browser, they see a one line error:
"An error occurred: FlowExecutionException"

Can somebody please help me decipher what the idp-process.log error
means and how I should go about addressing it?

Thanks!
Bryan