GielS,
Thanks for letting us know. Next, what happens when you Sign in and use iCloud.com to create a new note? For example, when you do that, does the note appear on your iPhone, Mac, or both?
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We recommend this step because it helps to isolate further so that we can find the root cause of your notes not syncing.
Also, please check the link below to ensure all steps we're followed.
Keep your notes up to date and share them with iCloud
To perform the recommended troubleshooting steps via the link above, complete the steps in the section titled "Access the same notes on all your devices."
When you set up iCloud for Notes, your notes are stored in the cloud instead of locally on your device. You can see them on any device that’s set up for iCloud and Notes, including your iPhone, iPad, iPod touch, and Mac. You can also access your iCloud notes in a web browser.
Because your notes are in the cloud, changes you make on one device—like creating a new note, deleting a note, or editing the text—automatically appear on all your devices. You see the most up-to-date version of your notes, no matter where you access them.
Please let us know the outcome after giving the above steps a try.
Thanks for using the Apple Support Communities.
Cheers!