Check on as many other forums as possible, including ones not made with PHPBB. If the problem persists, Safari may have a problem. Update it anyway.
What are you running safari on anyway? The iphone?
Oh, yeah, if there is no update, contact the makers of safari and give them info on the error, they may be able to help.
I think it is more likely to be a problem with safari than the forum. Although, there are strange things that go on in this forum...