Hi Dirk,
It’s not going to work that way either, because pages don’t have tags.
For example this page:
https://www.holidayomat.com/en/tips/canada-2017/ (page)
Does not have a tag or a category, because it’s a page, so the related posts widget displays based on other fields.
However for example, here the category match will work:
https://www.holidayomat.com/en/holidays-australia/au-2017/new-south-wales-2017/ (post)
Because this is a post, and it’s categorized, and all of the related posts under it are in the same category (australia 2017)