This is an issue that Spase needs to look into.
Essentially, it's an issue that occurs on vBulletin forums when the site experiences a high load - the forum blocks any new connections until the load has decreased and normal activity can be resumed. It can also be caused by the server hardware performing too many operations at once, such as automated backups. Since this seems to occur at a regular interval every night, I'm given to think that this has something to do with a high load on the server couple with the server performing routine backups, since this wasn't an issue prior to the forum upgrade and I seem to recall Spase mentioning a more frequent backup of the forum.
This is easily sorted by altering the 'NIX Load Limit' value in the server CP, and by ensuring the server hardware is running smoothly.