Having issue with startForPostWithGraphPath

Having issue with startForPostWithGraphPath

I'm really exhausted and couldn't no longer think straight. There seems to
be some problem that for the last 2 days, I no longer can post from my iOS
app. The app is still in development stage. The api I use is
[FBRequestConnection startForPostWithGraphPath:@"me/my app"
graphObject:action completionHandler:^(FBRequestConnection *connection, id
result, NSError *error)
An the error I'm getting is follow:
The operation couldn't be completed. (com.facebook.sdk error 5.) {
"com.facebook.sdk:ErrorSessionKey" = ", expirationDate: 4001-01-01
00:00:00 +0000, refreshDate: 2013-08-11 02:50:49 +0000,
attemptedRefreshDate: 0001-12-30 00:00:00 +0000, permissions:(\n
\"publish_actions\",\n \"user_photos\"\n)>";
"com.facebook.sdk:HTTPStatusCode" = 500;
"com.facebook.sdk:ParsedJSONResponseKey" = ( { body = { error = { code =
1; message = "An unknown error has occurred."; type = OAuthException; };
}; code = 500; } ); }
Is there a problem with FB since 2 days ago?
Much appreciated if anyone can shed some light.
Thanks,
BS