We sincerely apologize for the inconvenience.
I have checked your website, but I was unable to reproduce the issue during my testing. I can access the backend without any problems. For your reference, I have shared a screen recording in the hidden content area showing the steps I followed.
It's possible that I may have missed something. Could you please review the recording and let me know where my testing differs from the issue you are experiencing? This will help us investigate the problem further.
Thank you for your cooperation.