Hello,
I’m sorry to hear that you’re experiencing an issue with the 404 Redirection settings. It seems that when you enable the 404 Redirection and set the Fallback Behavior to “Redirect to Homepage” or “Custom Redirection,” an error starts to display on the accessible index page.
To address this, you can try setting the Fallback Behavior to “Default 404,” which should make the error disappear.
If you need further assistance with this issue or have any other questions, please feel free to let us know.
Thank you for bringing this to our attention.
Sources:
How to Fix the ‘Blocked Due to Other 4xx Issue’ Error » Rank Math
How to Find Out Where 404 Errors Are Coming From » Rank Math
How to Setup Rank Math SEO Plugin Properly
How to Fix the Submitted URL Seems to Be a “Soft 404” Error » Rank Math
How to Fix 404 Errors with Rank Math SEO Plugin
Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to this issue.
Please confirm you are using the latest version of the Rank Math plugin(https://rankmath.com/changelog). If you are and the issue persists, we might need to take a closer look at the settings. Please edit the first post on this ticket and include your WordPress & FTP logins in the designated Sensitive Data section.
Please do take a complete backup of your website before sharing the information with us.

It is completely secure and only our support staff has access to that section. If you want, you can use the below plugin to generate a temporary login URL to your website and share that with us instead:
https://wordpress.org/plugins/temporary-login-without-password/
You can use the above plugin in conjunction with the WP Security Audit Log to monitor what changes our staff might make on your website (if any):
https://wordpress.org/plugins/wp-security-audit-log/
We really look forward to helping 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.