Maintenance Page using V11

So again, "[LB::Customer_5013_5014_pool]" isn't a valid command. You can either use "[LB::server pool]" to expand the configured pool automatically when the HTTP_REQUEST event is triggered, or you can put in the literal string name of the pool:

if { [active_members "Customer_5013_5014_pool"] == 0 } 

So the following should be exactly what you need:

when HTTP_REQUEST { 
    if { [active_members [LB::server pool]] < 1 } { 
        HTTP::respond 200 content [ifile get "Maintenance_Page"] 
    } 
}