On the first glance everything looks completely normal. But you have a strange first lap with 5 km but no power, HR and GPS data. Maybe that did confuse Strava. And the second "Maybe": If you crop the start of the ride so it begins at a later GPS point, maybe this strange lap vanishes and Strava understands the ride then.

Maybe it‘s not flagged by Strava but by another user (for what reason ever). You should be able to resolve that problem. If not write directly to Strava support.

Other users can't flag specific segments, only the whole activity.

You don‘t say! Just look at the activity: ALL segments are flagged … In fact the whole activity is flagged …

Ah, okay, Have not seen the first 5 km-Problem

Hm, I can not find any Problem in the first 5 km. All Data available in my view

@EsCalaToR_One - Are you seeing power and heart rate when you look at the lap summary for lap 1? Everything looks fine other than that part.  I could see that possibly creating a glitch/flag to the Strava filters leading to it being auto-flagged.  If it was flagged by another person, when I (another user) goes to look at the activity, I would see a big red banner saying the activity had been flagged.  I don't see that with yours, so it does appear that it was a system flag rather than another user.  Probably need to submit a support request to Strava to get it cleared up unfortunately.  

Look again, only the segments are flagged, not the whole activity.

That's what we see: No Power, no HR and no part of the grey area appears as selected even though 5 km should be the first chunk of the activity:

But when I hover over the second lap than a portion of the ride beginning at the start is highlighted:

When the cropping of some meters at the start doesn't help I would go with anchskier that this must be escalated to the Strava support.

But that‘s exactly what happens when another users flaggs an activity: ALL segements efforts do not count for leaderboards.

When the system flags it, you will see the excluded efforts. When another user flags it, we would see a big red banner on the activity saying it has been flagged.  We do not see that on yours, which means it was a system flag, not another user. 

very interesting. I can see all the Data 🙂 

Aber schau dir mal die erste Runde an! Und zwar links über „Runden“: Angeblich 5km, aber sie enthält keine GPS Daten. Ich würde den Anfang mal abschneiden (vorher ggf die GPX sichern). Vielleicht liegt es daran. 

TRANSLATION: But just look at the first round! On the left above “laps”: Supposedly 5km, but it doesn’t contain any GPS data. I would cut off the beginning (if necessary save the GPX first). Maybe it's because.

@EsCalaToR_One - Click on the "Runden" tab on the left.  That is where you see the data from each lap (runden).  When you do that, look at the first lap.  You will see what Jan_Mantau posted showing no power or heart rate for the first 5k.  If you drag your mouse over that first lap, it doesn't highlight any of your map.  When you drag your mouse over the 2nd lap, it highlights the first 5k of your activity.  That is showing that something is wrong with the file data at the start of the activity.  Using the "crop" tool to cut off just a little bit at the start may trim off the bad data and get the rest of it working properly.

I´ve cut alittle bit and now I can see my PR´s. Thanks everyone for helping me!