This surfaces around the six-month mark, once the novelty has worn off and the social circle still consists entirely of other newcomers. Worth knowing before drawing conclusions: Dutch people who move to a new city for work describe much the same wall, which points at how adult friendships form here rather than at any attitude towards foreigners.
Because most Dutch adults built their close friend groups in basisschool, middelbare school, or their studentenvereniging years, and those circles tend to calcify rather than expand once people hit their late twenties and thirties. It's not exclusion aimed at you specifically — Dutch people moving cities for work report the same wall. The friendships that do form as an adult here almost always come through repeated, structured contact rather than one-off socializing: joining a vereniging (sports club, choir, hobby association), volunteering somewhere with a weekly commitment, or a recurring class. Show up consistently for months, not weeks, and let the relationship build on the back of a shared activity rather than trying to manufacture closeness directly. Colleagues and event-based networking rarely convert into real friendship here; the club circuit does, slowly. It takes longer than most expats expect and requires more patience than in more socially fluid cultures, but it's not actually closed to you — it's just built on infrastructure (clubs, routines) rather than spontaneity, and you have to plug into that infrastructure rather than wait to be invited in.