If there is no obvious reason or explanation why there is a problem, it is likely that you have visited a perfectly legitimate site and may have a cookie placed in your browser that is causing a problem accessing other sites. Some cookies also interfere with authentication off-site via Raven.
The solution is to clear the cookies and cache and browsing history from your browser and restart the browser. Some browsers also allow you to perform a total refresh of the browser if simply clearing the cache does not work for you. Wikipedia includes a useful artilce that gives basic instructions on how to clear your cache etc. in different browsers: Cache clearing and disabling.
