Dutchie here, FTA :

> wordt geklaagd over drugsoverlast, afval en naaktloperij

So the actual problems are complaints about drug users, illegal trash dumping and nudism. And the negotiations only took 5 years.

I had assumed that the land was occupied because the motivation for the exchange mentioned in the article are reports of crime (nudity, drugs) that Belgian police can't easily attend to. It is likely the suspects were visitors to the area.