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")

I have found the problem.
in the configuration of the adapter I set the server address to http://mandrillapp.com. I changed it to mandrillapp.com (without http://) and it worked!