We got an issue while we trying to open the site in SharePoint Designer we got an error “The Server Could Not Complete Your Request” as shown below.
We clicked ok and try to close, but it was repeatedly showing we open the Task Manger and in Application Tab we ended the Designer Session.
Open the IIS
Enable Anonymous Authentication for Your SharePoint Web Application
Select the Web application and Click on Authentication
Enable Anonymous Authentication as shown below.
For more information about this issue we also check this article