The site's system limits stories access to 1 story per user per 20 seconds. When you open too many tabs too quickly, the first one opens normally and the rest will have 'Error! Too many concurrent accesses' error message.
When you see the message, make sure you haven't loaded a different story in the last 20 seconds and then click the reload button on your browser and the story/chapter should load as usual.
Please keep in mind that the error message contains code that will reload the page after a short interval.