I kept receiving 401’s using credentials and code which worked fine on other machines. After some Googleing I found a thread on this site which spoke of clock flaws which might also throw a 401. This is not very obvious in my opinion, so I would suggest adding that as an option over here:
It might also be good to point out (more) clearly that being blacklisted means receiving no response at all (so not even a 401). That was an option I considered as well, even though I couldn’t think of why I would be blacklisted.