Static Maintenance Page

Sounds like you're missing a clientssl profile or forgot to enable automap. See below a working example from my lab environment.

Virtual Server:

ltm virtual vs_html_static_page {
    destination 10.23.98.13:https
    ip-protocol tcp
    mask 255.255.255.255
    profiles {
        clientssl-insecure-compatible {
            context clientside
        }
        http { }
        tcp { }
    }
    rules {
        A_IRULE_STATIC_HTML_PAGE
    }
    source 0.0.0.0/0
    source-address-translation {
        type automap
    }
    translate-address enabled
    translate-port enabled
    vs-index 219
}

irule:

ltm rule A_IRULE_STATIC_HTML_PAGE {
when HTTP_REQUEST {
    set html [ifile get "/Common/static_html_page"]
    HTTP::respond 200 content $html
    unset html
}
}