Hi,
When i have tried to open the symantec management Tab under Agent Plugin. It reflect error.
Oh weird...I got this exact same page today! In our case, turns out something maxed SQL memory usage and CPU utilization so we just rebooted the SQL server that houses our CMDB and all is well again.
Check the link
Check the article
Could you perhaps provide a screenshot of the Log Viewer error related to the above error.
Or you could just review all the comments above, suggestions are good and normally works.