Thank you for contacting Rank Math and sorry for any inconvenience that might have been caused due to that.
It being shown in the 404 monitor means something is trying to access those URLs but they are not found.
Assuming you are using the latest version of all the plugins and the themes (please update if not yet), it could be a conflict with one of the plugins or the theme you are using (something adding that username somewhere in your links)
It is fine, if you want we could take a closer look at the settings. Please edit the first post on this ticket and include your WordPress logins in the designated Sensitive Data section.
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: