Clients can't register with task server

Notification server name: dept-deploy

Site server: dept-deployss, running Task Service and Package Service

Starting about a week ago, clients are refusing to register with the task server.

Altiris Log Viewer:

Error loading document: The element 'html' is used but not declared in the DTD/Schema.
 (-1072898035)

Could not register using "https://dept-deployss.dept.example.edu:443/Altiris/ClientTaskServer/Register.aspx"

Thoughts?

Thanks!