Tips for keeping your future streak:

• 1) Use the tips in this post
  https://www.duolingo.com/comment/24771557

• 2) Losing a streak is a common complaint from people using the Duolingo App

(For users of the web version (www.duolingo.com) I see complaints from people, who are using outdated browsers and/or operating systems.)

It's always a good idea ....

• to check that your phone / tablet has had a chance to synchronise with Duolingo's servers.

Why would it not synchronise? My Wifi/4G/LTE is okay

Because Duolingo has issues in synchronising, almost every day.

Workaround
Always check if your phone had synchronised with the Duolingo servers after doing a lesson. Sometimes it can take 5-10 minutes after finishing a lesson in the App.
E.g.

• have always Duolingo's web version open in the browser of your phone
• set this browser in Desktop mode
• refresh your browser after each lesson in the App, and check if the amount of XP increased in the browser with the web version.

I suggest you edit this post and move it to the troubleshooting forum rather than the German forum. The German team have no technical access to help with this.

2 reasons. Either you are not meeting your daily streak goal. Or, eventhough you have met the goal and didnt connect to the internet after completion. Hope this clears your question. Good luck.

Do you have a streak freeze? It only costs 10 lingots, and safeguards your streak if you miss a day.