Vulnerability Blocked.....521 pending

A couple of weeks ago I started getting the symantec notification pop up stating vulnerability blocked. The tech at my school (I'm a teacher) ran the full scan and that seemed to get rid of the pop up. Today it started again. I tried to run the scan again but when I clicked "full scan" nothing happened. I minimized the window to make sure it wasn't happening in the background and it wasn't. It started a little over an hour ago and I'm up to 525 pending now. I attached a screen shot. What does that pop up mean, how do I get rid of it, and can I do anything to prevent it from popping up in the future?

Thanks

-Lisa