Problem: the category tag for the default language (NL = dutch) is called: category, and the category tag for the secondary language (EN = english) is called: categorie. It should be the other way round.
Solution: Go to WPML -> String Translation and search for "category" in WordPress domain. You'll see there is a "events/category" string that is registered as being in Dutch. Select it.
At the bottom of the tab, you'll see a button "Change the language of the selected string". Now change the language from Dutch to English, and then translate the string into Dutch as "events/categorie".
Save, then go to Settings -> Permalinks and resave permalinks again.
