Created an account here just to say that this worked for me.
I usually use 3utool to instal Phoenix 5.0 on my old ass iPad running iOS 9.3.5. However, 3Utools is giving me the error “jailbreak under maintenance”.
so I downloaded sideloadly and tried using that to side load a copy of Phoenix 5.0. I was getting the “unable to sign into Apple ID” even though all of the information was correct.
I simply went into iTunes, authorized my computer by logging in with that Apple ID, left iTunes open, opened iCloud and left it minimized, and retried installing Phoenix 5.0. It worked this time.
thanks for the tip. Hope other find this.