So the way I see it is fundamentally there was an issue with receiving signals on the spacecraft and that caused issues. I'd really like to know more about that. They mention doppler shift but that's bidirectional so even without the spacecraft knowing how fast it's going, they should be able to account for it based on the received signal. Common issues could be reduced receive sensitivity, interference, oscillator drift or instability, or plenty of other things but there's no mention of even one that I've been able to find.