I would think that to be highly unlikely.
I'd also think it unlikely that your forum's cache is corrupt, too. Might just be worth emptying it, though, just in case.
Have a look at Admin>Server settings, again.
Only one of these should be checked:
Enable local storage of cookies
(SSI won't work well with this on.)
Use subdomain independent cookies
(turn off local cookies first!)
If only one IS checked, try swapping them over.
On the same page, you'll see "Use database driven sessions".
Try changing that, too.