[Wesley Blokker]

For those who are interested; my question was solved on the Adobe forum.

The problem was with the pixel grid.

Begin quote:
Open the Transform panel (under Window > Transform OR press Shift+F8). Expand the Transform panel fully by clicking on the up-down arrows next to the name Transform & then uncheck the option ‘Align to Pixel Grid’. Turn off ‘Align New Objects to Pixel Grid’ from the panel’s flyout menu.

Any time you create a Web profile document, this option is turned ON by default – check in the New Document dialog, under Advanced section. If your document is not intended for the Web, you can turn this option off.
End quote.

Many thanks to CupertinoFruit.

Time is a valuable teacher, unfortunately it kills all its pupils.