Ankiweb : "Bad Request"

I would sometimes get an error like this if I was reviewing a card right at the end of Anki’s day.

In Tools / Preferences / Review, you have “Next day starts at: _____ hours past midnight”. So if I started reviewing a card a few seconds before the end of a day, and then marked it Good a few seconds after the end of the day, then this error would appear.

If by any chance this is what happened to you, then the solution might to be change “Next day starts” to some time in the middle of the night while you’re sleeping.