Widget suddenly displays “No data available yet.”

I recently realized on one of the websites I maintain, that the widget suddenly display “Noch keine Daten vorhanden.” (which roughly translates into “No data available yet.”) instead of the most popular posts. However, it used to work correctly before.

I set the widget with the following options:
– Show 5 posts
– Time span: last 30 days
– Post types: post

When I chose “All time” instead of 30 days, or “individual” with something >200 days, the widget shows the posts again.

Is there a minimal number of hits for posts to appear in the widget?

Checking our analytics tool, I can see that newer posts also have visitors.

Thanks for your help!