The fraud revolves around small companies ("gastouder kantoren") thinking they found a loop hole in the law. The short version is they would say: "Hey, do your (grand parents) sometimes baby sit your kids? We'll register them as a 'gastouder' company and now the government will pay for the childcare.". The problem was that these "companies" didn't have things like a proper administration which was required to get the government money. The "gastouder kantoreren" would them make up some fictional administration and get caught.
> the reality is much simpler: the Dutch tax office systematically engaged in discrimination, persisted in doing so when this was pointed out to them and made extraordinary efforts to hide their failings rather than to fix it.
You are framing this way too simple. The law which mandated them to come down hard on this fraud was written to combat the "Bulgaren fraude" (i.e. the Bulgarian fraud). In which east Europeans (mainly Bulgarians) would come to the Netherlands, claim benefits they weren't entitled to, get paid and then disappear. There was an public uproar and politicians (like Pieter Omtzigt) demanded stricter laws and hard enforcement of those laws to combat this.
> persisted in doing so when this was pointed out to them and made extraordinary efforts to hide their failings rather than to fix it.
They interpreted the law (which governed their actions) that it required them to come down hard on on fraud. The court initially confirmed (in a ruling) that this was the correct interpenetration of the law. It wasn't until much later that the court reversed on that position.
> If not for a single lawyer (González Pérez) persisting this whole thing would have never been brought to light.
It's worth mentioning her husband ran such a "gastouder kantoor", which was also investigated. You can look up the court documents online. It was quite clear he was running a fraud because for example all the hour sheets for the "gastouders" were the same.