Am Romanian, reading how the author was robbed on the bus because he had the invalid impression that because it was a Danish bus there would be no thieves in it almost made me start out laughing. One of the first reflexes I developed when I moved to Bucharest as a freshman student almost 20 years ago was to always check my back-pocket for my wallet (and later my front-left pocket for the phone). I still do that gesture up to the present day. I could also tell you how about 10-12 years ago a woman tried to convince me, my ex-wife and my ex-MIL that her son was badly injured in a Bucharest hospital, about to die, and that she needed some money in that very moment, she was crying out in the middle of the street just in front of us. My ex-wife would have given said lady money, but something seemed suspicious to both me and to my ex-MIL and we didn't. We returned to the same area about 15-20 minutes later and said women was still in the same place, this time not crying, calm, waiting for another "victim" to pick.
Not trusting people by default is not bad, it's an evolutionary trait that has helped us as a social species over the millennia. I'm just reading a wonderful book that it's based on not trusting people and which, nevertheless, has had an immense positive influence, I'm talking about the collection of articles called "The Federalist Papers" by Madison, Hamilton and Jay.
Even if all the groups came from high trust countries, if combined they do not feel a sense of shared values, patriotism, they will have low trust between these groups.
The problem comes from the recent wave and semi-open borders within EU that make it easy for some mafias to move into the country to commit crime. It's a tought problem.
I met a group of young, liberal, well-educated Danes on a trip to Japan once and later, accepted their invitation to visit Denmark, where I stayed for about ten days.
Make no mistake - these were picture-perfect Copenhagen Danes with exactly the liberal and progressive bent that you would expect and they were quite well educated and relatively wealthy (one of them even had a car!).
These people did not beat around the bush: they can't stand "the turks", they don't understand why they are there or why they behave the way they do and they cannot believe that their society has been infiltrated in this way.
It wasn't hateful, or even racist per se ... just a very pragmatic rejection of events.
This was in 2004 - I can't imagine they feel any better now.
Of course, in his eyes he was not an immigrant, but an expatriate.
A little of effort trying to understand others can make wonders. Most immigrants behave, a few not. Of those, a few are just bad persons, but most are just poor an uneducated and probably they would prefer to be in their home countries.
If immigrants are so problematic, maybe we should be looking into what, in many cases, our governments are doing (or not doing) in their native countries.
Both left and right leaning governments have failed to implement good integration policies. The parties on the left have been to naive and assume that things would naturally work them selves out: They trusted that the people coming into the country would want to learn the language, integrate and become Danes. On the right the opposite was assume, that the policies should be to not let people feel welcome, in the hope that they would leave.
Troublesome groups are let in, because Denmark is legally required to. But the evaporation of trust in others isn't purely an immigration issue, it happens between Dane as well, and that's worse because we're not entirely sure why it's happening.
Unfortunately we've had a series of governments with increasingly bad integration policies, leading to ghettoes and increased friction.
I'm reminded of "The Evolution of Trust" [0], a fun and interesting online simulation of agent-based cooperation with iterated Prisoner's Dilemmas. Of course no society is perfectly uniformly cooperate/cooperate, but some have an equilibrium that is more in that direction than others. It's a property that can be eroded by the exploitation of group norms you describe, and the system has to adapt by assuming more malice on the part of others.
I wonder if anyone has written deeply on the trend toward externalizing and formalizing implicit/informal trust structures. Of course it's not a new thing (think of Renaissance-era credit systems that were arising that allowed for wide-ranging merchant trade in the absence of face-to-face individual trust), but the rise of the police state, cryptocurrencies, and even the new Amazon Go stores that outsource anti-theft trust to computer vision all seem to be an increasing trend toward assuming that informal trust is impossible in our new environment and that alternate formal systems are necessary.
As the article points out, we're struggling to absorb outsiders. Teaching a Romanian immigrant, or a refugee from the middle east that it's completely safe to trust others is extremely hard. Likewise when an outsider break that trust Danes become extremely upset and hostile.
We're also becoming increasingly distrusting of other Danes, as people seem to become increasingly self-centred. This was a debate in a radio show a few weeks ago, sadly the reason wasn't discussed, only the danger of losing trust. Trust is hard to build, and our government is increasingly looking towards control and surveillance to ensure safety instead. Stores are moving certain items up to the register, because thieve simply walk out with entire boxes of Nutella or instant coffee.
As a Dane this is extremely disheartening to watch. The feeling that the safe, stable, equal and embracing society, you grew up in, is slowly evaporating isn't a pleasant one.