What's the reason for the board is temporarily unavailable?

[Xor]

Dear Administrator:
What's the reason for the following error that frequently appears after logging into the forum:

Information
Sorry but the board is temporarily unavailable, please try again in a few minutes.

[Enigma]

Ddos attacks, sorry for that...

[sarahdavis]

DDOS attacks explain the "board is temporarily unavailable" message well — when server resources are exhausted by a flood of requests, the forum can't process legitimate logins, so the error appears intermittently. Since it happens frequently after logging in, it may also be that the protection layer rejects or queues some requests during an attack. Nothing much users can do beyond retrying after a few minutes.