Hi Hendrik,
Where did you disable this setting? Please check in these places:
1/ Menu item of that page
2/ Backend >> Articles Options
3/ Backend >> Content >> Articles >> open that article settings
The setting from menu item will be used in priority.
webmaster omroepmuseum Additional question: can one change the word "BY" before the author by something else (by preference Dutch!)?
You can override this language constant: TPL_CONTENT_WRITTEN_BY
Check out this guide for how to override language for Dutch: https://docs.joomla.org/Help4.x:Languages:_Overrides