Do you remember what you did before the error occurs? If you've added an extension or changed the settings in your Web browser, try reverting to the previous configuration and disable your extensions.
Make sure your Web browser is on the latest version and clear your cookies and cache if you haven't done it yet. Then, restart your browser and try logging in to your account.
Also, a poor network connection may also be the cause of the "error 15" message. Try testing your network speed by going to SpeedTest. You can also try restarting your router in case the results seem slow, this may help.