I can replicate this problem too, although it seems to be intermittent. Often when an IT problem starts occurring "out of the blue", it's actually because of some change made by the IT staff. In this case there's only one IT staff member, and I did recently change some things.
People were having similar problems three years ago, after I relocated the forum from the chihuahua-puzzle.com server to lexigame.com. It seems I found a solution then, but I don't recall what it was. Hopefully I'll work it out before long.
In the meantime, you should always be able to get to the forum by clicking on the Forum menu button at the very top of the page.