Hallo Joris, kijk even verder want er is ook een Nederlandstalig gedeelte hier op het forum.
I think your problem (or similar ones) is/are discussed here before.
Just take time and browse this part of the forum.
Maybe your board doesn't reset, which is required to load a sketch.
If that is the cause of your problem, you could try resetting your board by hand at the start of your upload.
And as stated in the first line, there is a Dutch part too where you can ask your questions in Dutch if English isn't your thing.