Add the following XPath to the list
[ Advertentie_Gebruiker/Gebruiker/Gebruiker_Account ‘[%CurrentUser%]’ ]
I already see your issue.
At first yu domain model isn’t proper suited for yuor request. You do store the name of the Gebruikers in the match entity.
It would make more sense to have two associations between the Match and the Gebruiker entity. One to define gebruiker1 and one to define gebruiker2. These associations should be populated at the moment you now set the names as attribute value.
With both association you now can constraint it to gebruiker/account