Hi @Phil Burkhalter 

Does the "class namespace" mean package? (Sorry if I'm wrong.)

If it's correct, you can get class name (=package.class) using WHERE clause to INFORMATION_SCHEMA.TABLES.

Query example: I can filter "Test" package.

SELECT CLASSNAME FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA='Test'

Hi @Jani Hurskainen 

It seems the "UserErrors" domain name is the default name of user-defined errors, although it isn't written in the document.

I tested below:

<?xml version="1.0" encoding="UTF-8" ?> 
<MsgFile Language="en">
  <MsgDomain Domain="UserErrorsABCDEFG">
    <Message Id="-911" Name="MyError">Test!!An error has occured</Message> 
    <Message Id="-922" Name="MyError2">Test!!An error has occured 2</Message> 
  </MsgDomain>
</MsgFile>

set x=##class(%MessageDictionary).Import("/src/test.xml")
set error=$system.Status.Error("<UserErrorsABCDEFG>-911")
set error=$system.Status.Error("<UserErrorsABCDEFG>-911") 
write $system.Status.GetErrorText(error)
//ERROR <UserErrorsABCDEFG>-911: Test!!An error has occured

If you don't add default domain name, you can get the correct user message by specifying the domain name with the error id to create the error messsage.

I guess you had set [Interoperability] - [Service Config Name] in your FHIR repogitory like this:

When I start below production on same namespace with FHIR repository, I can get same error in your question. (I didn't set "HS.FHIRServer.Interop.Service" in Service.)

I can make REST requests if I change my production in this way.