Go to Reports section. Here you will see logs of all events on the workstations, start and end times, from where was the Task initiated ( client side, or server side ), and the Summary reports.
The Tasks that finished with errors are marked in red. The Tasked that finished with Warning are marked in orange.

You can sort the list by any field, and filter the list by computer name, task name, date, status and event type.
The Administrator should load the Warnings & Error Reports page, check the summary reports for each event and correct the problems, and restart the Task if necessary. For Tasks that became unresponsive, and the Workstation stopped sending messages, the Business Server will mark those as errors after 5 minutes of inactivity.