When starting a task, the devices stay in pending and the task stops.
If you analyze the file ScheduledTaskHandler_<#TASKID>.log, you see the following error:
Failed to initialize the RAXfer libary (3)
The Core server private keys and certificates are missing and secure communication cannot be established between the Core and the clients.
Restore the folder \Program Files (x86)\LANDesk\Shared Files\Keys\ and all the files it contains.