Hello,
I apologize for the issue you are facing. The error message you received, "Chyba 400: invalid_request," indicates that the out-of-band (OOB) flow has been blocked for security reasons. To resolve this, you will need to follow the Out-of-Band (OOB) flow migration guide provided in the developer documentation.
Here is the link to the developer documentation for more details: Súvisiaca dokumentácia vývojára.
If you encounter any further difficulties or have additional questions, please feel free to reach out to our support team directly by clicking here. We are here to assist you at any time.
Thank you for bringing this to our attention. Have a great day!
Sources:
Why Am I Getting "API Error: HTTP 400" Error » Rank Math
Sorry, You Are Not Allowed to Access This Page » Rank Math
Hello,
Thank you for contacting us and sorry for any inconvenience that might have been caused due to that.
Please check if there’s some security rule inside your .htaccess file which may prevent saving the settings.
You can also use the default .htaccess file from here: https://wordpress.org/support/article/htaccess/
Please do take a complete backup of your website before modifying your .htaccess as it is a sensitive file.
If the issue persists, please get in touch with your web host and ensure that the ModSecurity is disabled or they are not blocking any process of the plugin.
Let us know how it goes. 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.