Quick solution. Went into Task Manager, selected to show processes for all users.
It showed that 3cxBackup.exe was running under Admin. I am logged in as Admin, but it seems because the Task runs from task scheduler, it does not show up here.
So, I terminated the process and backup is working again.
So, the task was hanging. It was set to terminate - but after 3 days. I changed that down to 2 hours.