Hello,
Assuming you are using the latest version of all the plugins and the themes (please update if not yet), it would seem like a conflict with one of the plugins or the theme you are using.
To determine that, please follow this troubleshooting guide:
https://rankmath.com/kb/check-plugin-conflicts/
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.
Dear Sandeepan,
First thank you very very much for your prompt reply.
I found the mistake: after re-loading my Mac operating system, it included an old version of the Safari browser.
Updating the browser solved the issue.
There has never been a problem with Google Chrome,
Thanks,
Jürgen
Hello,
Glad that helped.
Please feel free to reach out to us again in case you need any other assistance.
We are here to help.
Thank you.