Custom Taxonomy specific to a Custom Post type – rewrite URL

Well, from what I can see, the reason for your wrong middle part is that it is hard coded in the slug?

'slug'                  => 'behandelingen/behandeling_categorie',

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)