Hello,
We apologize for the issue you are facing with the 400 error. This error typically indicates that the server cannot process your request due to a client-side issue. Here are some common reasons and potential fixes:
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Website Under Maintenance: If your website is under maintenance, it cannot be accessed. Please ensure your website is back to normal mode.

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Check Website Status: Ensure your website is returning a 200 status code. You can check the status of your website here.

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API Timeout: If your website takes more than 30 seconds to load, it may result in a timeout.
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API Requests Blocked by Firewall: Your server’s firewall might be blocking our API requests. You may need to whitelist the IP 193.138.6.5 and the user-agent RankMathApi/2.0.2; https://rankmath.com.
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Locally Hosted Website: The SEO Analyzer module of Rank Math only works on live sites that return a 200 OK status when crawled from an external site. If your site is hosted locally, the SEO Analyzer will not work and will return an API error.

If you continue to experience issues or have further questions, please reply to this ticket, and a human agent from the Rank Math Support team will assist you further.
Sources:
Why Am I Getting "API Error: HTTP 400" Error » Rank Math
Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to that.
Could you please confirm where you’re facing this error? You can also share a screenshot with us by using a tool like https://imgur.com/upload so that we can check the issue further for you.
Looking forward to helping you.
Thank you.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.