Some pathways in arc diff end up engaging a file uploader flow, but the flow now relies on a ConduitEngine rather than the older ConduitClient. This can produce an error like this one:
EXCEPTION: (Error) Call to undefined method ArcanistFileUploader::setConduitClient() at ...
I suspect the cleanest fix is to have the fallback mode of arc build both a Client and an Engine; this is likely straightforward.