It seems that the eagerly awaited trial for the newest campaign, and expansion to the Guild Wars franchise has been delayed due to a technical issue:

The start of the Nightfall World Preview Event will be delayed by a few hours. We’ve discovered a corruption in the archives, and we’re pulling a full backup, after which we will then examine the error, make a correction, and the event will be on its way. At this point, we anticipate that the event will not start before 5:00 a.m. PDT (Noon GMT). We will update as soon as we have further information. Thank you for your patience and understanding.

It isn’t easy to pull a live worldwide event, we witness those difficulties in the console market, and we’re witnessing them now. Let us just hope the rest of the event goes smoother.