Just on the off-chance, have you logged into the forum, then clicked the 'log out' link, closed your browser and logged back in again? That operation clears the forum cookies (rather than deleting all your cookies) and might solve your problem.
I'm using Firefox at the moment, although until my Windows laptop died last week I had been using IE8 with no problems.
Failing that, I have seen problems on one other forum (that I used to admin) where users could end up in a similar position (PM works, but some forum viewing doesn't and posting doesn't) and that was solvable by manually changing their usergroup, saving their profile and then changing it back again and re-saving. I'm not suggesting that's the case here as it always happened as a result of a paid subscription elapsing, but if you get to the desperation stage, it might be worth a go.
Ian.