OK, found issue: it was caused by the Evernote Webclipper. It seems as if there is a conflict between the 2. Disabling that extension and the ads are shown.
Strange, but perhaps something to have a look at.
Using Adguard Safari Extension, and disabling protection on https://tweakers.net.
No luck: no ads are appearing.
When reloading the page without content blockers, the issue is gone and the ads are visible.
Thanks for having a look.