That's utterly bizarre
I have no idea why you're having problems. I've checked your account, and you're in the correct group, and also I've checked the rights that the group has in terms of forum access and they are correct.
I'm wondering whether it's worth creating you an entirely new account, see how that works, and merge your existing one onto it. I'll make an account in just a mo and see if I run into any problems with forum access when in the same group as you. If I don't, you can try the account yourself, swap between the two and make sure that you don't have problems either, then merge. Are the problems occurring while accessing the site from different places (ie school, home etc)?
I'm a bit concerned that it has happened in areas that you were able to access. I'll check with Lorian tomorrow as well to see if there were any more server issues, as it might just be that you had a case of really bad luck and managed to hit both times that it was having problems.