For what it's worth : One, i deleted the sandbox account. It had the exact same name and credentials, maybe this was messing stuff up in the magical PayPal cloud. This seems to have had an effect (not sure exactly what but there was a difference). Two, whenever I get the 400 error, I just reload the page and retry whatever I was doing. It mysteriously works the second time. So for instance, if I got the error when I was logging in, I just click the back button and log in again. Works. This is really an «interesting» problem, and not in a good way.
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