Thanks for your response, but this is weird.
Because I always get "Http/1.1 Service Unavailable" on my iPhone Safari, and delete the app then install again, can re-trust developer as normal.
My customer also get "Http/1.1 Service Unavailable", he can delete, re-install, re-trust app. BUT just cannot verify the app.
BTW, did you have different messages from https://ppq.apple.com ? If so, could you kindly share with me?