Create an empty list first, then retrieve the medewerkers and add them to the empty list. From there you should be able to add and subtract.
My theory: Realize that this microflow is converted into Java code. With that in mind, perhaps the instance of the current user is a different object in memory than the one you just pulled from a Retrieve statement. So a standard remove from list may not work (they'll have different object ids in memory - which is the default way to compare objects in Java).
An alternative to doing a Remove (like in your microflow) :
1) Ensure you have some sort of unique identifier, like the username, AutoID, etc.
2) Perform a Find, on that attribute.
3) Then remove the found object from your list.