I've just bought JFacebookConnect for my Joomla 1.5 site but it doesn't work. Although my app is set up properly in the backend, when I try logging in or registering using the FacebookConnect button I get either a blank page or a 403 error alert. In Google Chrome I get a server error alert recommending to try again later. Strangely, the Facebook connection seems to work when I'm logged out of my Facebook account and I'm prompted to provide my details but when I'm logged in the error occurs again. I've tried disabling and enabling again all of the active plugins as suggested in previous posts, but I'd avoid setting the Joomla error reporting to maximum, because this may disclose sensitive information (SQL path and password etc.). Please help, thank you in advance.
You'll need to enable Error Reporting, just temporarily while you test, so we can get a better idea of where the problem is coming from. I did just look at the site listed in Autotune, and it doesn't seem like the JFBCSystem plugin is enabled at all. I'm assuming you disabled it right now, but if not, that's probably part of the problem as well.
Let us know what error you're getting. Beyond that, check if you have any Firewalls or other security extensions installed and try disabling them, again temporarily, to see if that helps the problem.