The Activity table is populated by the TeamPulse Integration Service. Could you please check if the Integration Service is up an running? You can open Control Panel -> Administrative Tools -> Services and find TeamPulse Integration Service. If it is not started, please start it again.
If this is not helping, I would ask you to send us your Log files, so we could investigate this issue. They are usually located here: "C:\Program Files (x86)\Telerik\TeamPulse\Logs".
I'm looking forward for your answer.