[Gelöst] Reappearing development site banner

Background of the issue:
I am trying to permanently remove the development site banner. The banner can only be removed by clearing the site cache, but this solution is not permanent, the next day the banner somehow appears again. Link to a page where the issue can be seen: versteckter Link

Symptoms:
The development site banner reappears on the bottom of the page after being removed by clearing the site cache.

Questions:
How can I permanently remove the development site banner?
Why does the banner reappear after clearing the cache?