1) Close the window that has the error
2) Log out
3) Clear your browser's cookie and cache ( make sure that you have logged out of all your online accounts before you do this part )
4) Close the browser
5) Count to 5-10sec, then re-open your browser and log-in again
Other things to check:
a) Make sure that you have all the necessary updates ( plug-ins / add-ons like Adobe Flash, JAVA )
b) Make sure that there's no other program/add-on/plug-in that prevents or blocks the actions of Flash
c) Periodically clean your browser every time you completely log out by using CCleaner ( at least once a day )
d) Periodically scan your gear ( desktop / laptop ) when you're offline ( make sure that you have updated them )