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I’ve run into similar OAuth 400 issues before, and clearing the Instagram connection inside the module settings and redoing the app permissions from scratch in the Instagram account usually fixed it. Sometimes the token gets revoked without warning. I’ve had to reset mine a few times, even though I was just using features like how to see what someone likes on instagram for my own checks. Also make sure your server time is synced, since that can break the handshake.
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I’d try generating a new access token in your Instagram settings, then clear the module cache in the theme. Sometimes the old token expires and triggers that 400 OAuth error.