Why was this man in an open prison?

[Dilbertian]

https://www.bbc.co.uk/news/uk-england-lincolnshire-60372887

A "dangerous sex offender"... serving a life sentence for attempted rape and indecent assault...presents a particular danger to women and young children ... can cause real harm to anyone he comes across.

Why on earth was he in an open prison and not in a high-security prison?!

I doubt anyone here can answer for this specific prisoner, but there is no bar to prisoners who have committed serious offences being moved to open prison. A prisoner can be moved to open prison if the parole board believes they are not a danger to the public or other inmates and that they are unlikely to abscond. Clearly the parole board will get some cases wrong - some prisoners will be transferred to open prison who are flight risks and/or dangerous, whilst others who could be safely transferred are not.

In this case, clearly the parole board was wrong about whether he was a flight risk. Also, the police clearly disagree with the parole board about whether he is a danger to the public. Given that the parole board got the flight risk wrong, it is reasonable to think that they may also have been wrong about the danger to the public, but it doesn't necessarily follow.

Aren't recidivism and opportunism particularly high among violent sexual offenders?

I don't know about opportunism but there are various studies that suggest recidivism is actually lower than for other types of crime. Even if such offenders do re-offend, it is not usually a further sexual or violent offence.