Hello,
It seems like a prpblem with app_offline page....
check out the following...
http://www.chadgreen.com/post/2009/04/04/Taking-An-ASPNET-Web-Application-Offline.aspx
http://yaneshtyagi.spaces.live.com/Blog/cns!F66AE167494DDA0!293.entry
And if it still not that problem then contact your host and may be i think he can fix it...if still the problem persists then do let us know to find out more solutions for the problem...
Hope this helps you...
Thanks.