Not-Found and WP Defender Issues

I am having issues when trying to access the admin dashboard of my sites. I chose this one to reference because I am also not able to update plugins from WPMU dashboard on this site as well.

When I try to connect using the link within the WPMU portal, I get redirected to the not-found page? Not sure why it happens, but it's doing it a lot.

In addition to this, the Defender plugin is generating a lot of server-level errors. So much, that I am unable to use it on any site.

As I really enjoy the service you provide, how can I fix these issues and get things moving smoothly again?