URi based redirection only if pool members are available.

So you have 3 members:

member1:8083 - Error page member1:8003 - Normal page member2:8003 - Normal page

Then you should be able to keep your priority group activation, or try the node command?

when HTTP_REQUEST {

    if { [active_members http_pool] >= 2 } {
        if { [HTTP::host] equals "example.com" and ( [HTTP::uri] equals "/" or [HTTP::uri] equals "/example-client" or [HTTP::uri] equals "/example-client//" ) } {
               HTTP::redirect "https://example.com/example-client/login"
        }
    } else {
        HTTP::uri "/errorpage"
        node member1 8083
    }
}

Please note that this would require you to remove member1:8083 from the pool and disabling priority activation.

/Patrik