Okay, I found my error. Somehow my catch-all construction had become empty instead of triggering an Ens.Alert....

I figured out there is a function called SendFormDataURL where you can include the tURL as the first parameter:

set tURL = ..Adapter.URL_"login/login"

set httpRequest = ##class(%Net.HttpRequest).%New()
    set httpRequest.ContentType="application/json"
    do ..ObjectToJSONStream(pRequest,.jsonStream,"aeliw")
    set httpRequest.EntityBody = jsonStream
    set tSC = ..Adapter.SendFormDataURL(tURL,.tHttpResponse,"POST",httpRequest)

This was the answer:

I resolved this by adding the code to the Business Operation class instead:

 Set header=##class(SforceService.tns.LoginScopeHeader).%New()
 Set header.organizationId"MyOrganizationId"
 Do ..Adapter.%Client.HeadersOut.SetAt(header"SforceService.tns.LoginScopeHeader")