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)