Did you resolve this? I'm having the same problem. I tried exporting my includes files. It uploaded 50 right away, and now I keep getting the request failed (retry in X seconds).
EDIT: 50 seems to be the stopping point for everything. I have tried exporting different folders, and even had W3 TC create a new bucket in case there was an issue with the one I was using. No matter what, it always stops after 50.