Fitness app not displaying data despite Health app collection on iPhone without Apple Watch

Since quite some months I don't get any data any more in my fitness app. I used an Apple Watch for some time and learned this will stop collecting data when just carrying the phone. So I decoupled it. Still no data.

The collection (settings) work fine as I see everything in my health app.

I learned you might have to remove additional devices from the health app so I removed everything but my current iPhone AND there is an additional iPhone stating "no data found" but no possibility to remove that.

Any idea what else to do- I'm not used to problems like that with apple :(

[Re-Titled by Moderator]

Original Title: Fitness App not showing any data any more, Health collects them, no Apple Watch