Re: Struggling to understand....
"I haven't seen reported what the actual problem was"
If you have time I strongly suggest you read https://www.private-eye.co.uk/special-reports/justice-lost-in-the-post (and then start buying Private Eye on a regular basis ;o> ). Basically it was a tissue of lies from the start: large and unexplainable 'accounting errors' in post offices all over the country that couldn't be possible without either massive, wholesale fraud or a serious software bug or bugs. The Post Office and Fujitsu maintained all along, including in multiple postmaster prosecutions, that Horizon was utterly infallible and especially that it couldn't be accessed remotely so all discrepancies must be fraud by the postmaster / mistress from within their office.
According to the PE report, internally the PO and Fujitsu knew their defence was a lie and some of each's witnesses knowingly perjured themselves, including at the big trial last year. The sainted Vennells did her best to keep at arms-length and maintain plausible deniability. Thus far she's effectively got away with it. Whether karma catches up with her remains to be seen.