We have had this rare fault reported before. We investigated it at length and found it very hard to recreate. However, our server hosting company believes that it is caused by their firewall overreacting to too many connections in a way that makes it trigger. 

We have found that waiting a for a period of time normally fixes the issue. Sometimes this can be an hour and sometimes a bit longer.

Sometimes it can also help to clear your browser's cache by doing a hard refresh of your browser screen.