Error: The page you are looking for is unavailable

[samergains]

Hi everybody,
I'm facing a problem with my forum which I guess is related to the server, but i'm not quit sure.
The forum gets unaccessible everyday for about half an hour and we get an empty white page with no error code just the sentence:
"The page you are looking for is temporary unavailable. Please try again later"
as I googled that I guess it's related to Nginx but i'm not sure what i have to do next.

any ideas plz

vb version: 4.2
Apache v2.2.15 over Nginx

--------------- Added [DATE]1350645794[/DATE] at [TIME]1350645794[/TIME] ---------------

some statistics about our forum:
members: 187,490
daily visits: 10~30,000
the problem happens in evening which is the most crowding time but i'm not sure if it has to do that.