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.
This happens to me multiple times a day, sometimes. Most of the time when I want to check for new threads, or check the threads that I follow, that message pops up. But it's not only when I get on, sometimes the site actually kicks me off when I'm in the middle of reading something, because "the server is too busy".
This shouldn't happen at all, unless your page is actively refreshing itself. The page you're currently on will be cached and nothing will happen until you enter another connection to the server - the issue is literally the server blocking new connections until the load is decreased.
Simply put: if you're reading a thread and the server experiences this error, you won't know about it until you try to load a new page.
I have no idea why it hasn't been fixed by now. I'm fairly certain that Spase SHOULD know about this issue and, now that I've said something on how it should be fixed, it should be a simple matter of going in and doing so.