Hello,
I apologize for the inconvenience you are facing with the Rank Math Plugin fatal error. It seems you have tried the solutions provided for the fatal error issue after the 1.0.49 update, but the problem persists.
If you have already attempted the suggested fixes and the issue remains unresolved, the next step would be to create a support ticket with Rank Math. You can do this by visiting the support page and clicking on Submit a Ticket as shown in the image below:

By submitting a ticket, our support team will be able to assist you further and provide more specific guidance based on your website’s configuration. Thank you for raising this concern, and we appreciate your patience as we work to resolve this issue for you.
Sources:
Fatal Error Happening on Some Installations After 1.0.49 Update » Rank Math
Does Installing Rank Math Slow Down My Website? » Rank Math
How To Effectively & Efficiently Get Support from Rank Math (Support Tickets) » Rank Math
Manually Install Rank Math via FTP (or SFTP) » Rank Math
Hello,
Thank you for contacting us and we are sorry for any inconvenience that might have been caused due to that.
Can you please share the error log with us?
Having the error log file will give a clear idea of why this might be happening.
You can follow this guide to enable debugging in WordPress https://wordpress.org/support/article/debugging-in-wordpress/ or you can get in touch with your web host, and they will help you locate the log file.
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.