What
Account device binding, which links your account to the machine you use, now goes through a check that asks whether a device proof is available. If there is none, binding fails with no_device_proof. If getting the proof itself fails, binding fails with that error.
Why
An account can no longer be bound to a device without a proof from that device. If binding fails, the no_device_proof error says why.
Something disagreesSomething we can check disagrees with this entry, or the writer said they could not settle it.
The writer flagged doubt
The finding does not say what a device proof is or how one is obtained.