This article is very hard to follow to figure out what has actually happened or what CumEx is.
In the beginning of the article is a rough description of the trade in question:
> Cum-ex – this is the name German media have given to this scam. Internationally the different variants of these trades are known as dividend arbitrage.
> Cum-ex and its variant cum-cum were highly complex share deals with no economic purpose other than to receive tax ‘reimbursements’ from the state – but for tax that had never in fact been paid. This is how it went. The participants would lend each other shares of major corporations, creating the appearance for the tax authorities that there were two owners of the shares when in fact there was only one. The bank which settled the trades would then issue a ‘confirmation’ to the investor that tax on dividend payment had been paid to the tax office – when in fact it had not. With this confirmation in hand, the investors were then ‘reimbursed’ by the state. It’s a bit like parents claiming child benefit for two – or more – children when there is only one child in the family.
Late in the article we get a description of the origin of the trade:
> The money machine
> According to Frey, an equity trader at a US investment bank came across the trade accidentally. He had bought shares that were delivered four days later. This interval covered the dividend payment day of the company whose shares he had purchased. This profit is taxed in the domicile of the company (say, Germany). German shareholders can ask for this tax to be reimbursed because they have already paid corporation tax.
> The trader suddenly realised he had this tax payable in his book without actually owning the shares. The amount was £50 million. It was a very large trade.
> The trader wanted to get rid of these funds that were not his. He approached the seller of the shares who had also been reimbursed his tax. The bank’s legal department sought professional tax advice to find out how to return the money to the tax office. The answer came back: “You can keep it.”
> There is no law that prohibits a multiple tax payout, they said. And if something is not explicitly prohibited, it is legal, the tax advisors argued. The trader kept the money. And because the tax reimbursement happened automatically, the scheme could be repeated time and again.
> All you needed was enough funds to trade the shares for a few days around dividend payment date. Or you could even just borrow the shares.
Following "dividend arbitrage", there are a couple of other articles that talk about it, Bloomberg [1] and ProPublica [2], though it's hard to tell if they're talking about the same thing or an older version of the scheme. In Denmark, where there is the most evidence of reimbursement for unpaid taxes, it looks like the investigations date to 2015 [3].
[1] https://www.bloomberg.com/news/articles/2018-05-18/denmark-t...
[2] https://www.propublica.org/article/denmark-is-big-victim-of-...
[3] https://www.thelocal.dk/20150925/danish-tax-authority-cleans...