I am having this same problem whenever I start my browser with an FM tab open. Selecting that tab refreshes it and I get the "Access Denied" message. I can go into the browser (Safari) settings and clear my cache, etc. for FM, then close the FM tab and the browser, wait a few seconds, then reopen the browser and open a new tab for FM and I'm able to log in again. But if I close Safari, or even use it for several minutes, the FM tab will randomly give me the "Access denied" message upon the next click on that tab.
I understand the site has had some problems with hacker attacks recently, but that the problem had been cleared up. However, it is continually frustrating to have to repeat the process every single time I use the site. I hope Fred (and his team?) can get this figured out.