Joseph McCann was released from prison after an error by the probation service.
One staff member was demoted after an investigation into the circumstances of McCann’s release found he should have been recalled to prison after he committed a burglary while on licence for an earlier offence.
Instead, he was handed a fixed prison sentence and released automatically half way through, without Parole Board involvement, in February.
McCann, 34, had been given an indeterminate sentence for the public protection (IPP) in 2008 after admitting breaking into and burgling the home of an 85-year-old man.
The terms of that sentence meant the Parole Board had to decide if it was safe to release him once his tariff expired.
Following his release on licence in 2017, McCann committed the second burglary and should have been recalled to prison, sources said.
But he was remanded into custody and then given a three-year determinate sentence, which did not take into account his earlier offending.
His release in February came two months before he began his sex attack spree.
Following his arrest in May, the Ministry of Justice carried out a serious further offence review into McCann’s case.
Dr Jo Farrar, chief executive of HM Prisons and Probation Service, has "apologised unreservedly" for failings in the McCann case.
She said: "These were appalling crimes and our thoughts and sympathies are with the victims and their families.
"We recognise that there were failings and we apologise unreservedly for our part in this. We are committed to doing everything we possibly can to learn from this terrible case.
"We have taken strong and immediate action against those involved in the management of McCann's case and are taking significant steps to improve intelligence sharing between agencies."
McCann's 15-day reign of terror
The review is understood to have identified an error in the probation service failing to recall McCann when he committed the burglary.
Of four probation staff in the South East and Eastern division who were directly involved in McCann’s supervision, one was demoted, according to PA sources.
An earlier investigation resulted in one member of staff involved with McCann being dismissed and an agency worker’s contract being terminated for "poor performance", although it was not understood to be directly related to the case.
It is understood the number of probation workers in the Hertfordshire area has increased by around 40% since September 2017.