I'm getting this message constantly. It probably is some unsupported software that was installed with a previous version of macOS, but I've not been able to find what is causing this. Looks like 'securityd' from the little information the logs provide.
Also, Safari is not working at all. I've removed all extensions (I think). Anybody experienced this?
(The image upload is not visible?) It is a modal dialog that states: Unapproved caller. SecurityAgent may only be invoked by Apple Software.
If you use multiple desktops, you can drag this monster to another desktop and do stuff with other dialogs. E.g. remove software from you HD that requires a psssword. This leads me to believe that this is some modal dialog generated by a piece of software that has a UI, like the Finder????