I just updated to Octopi 0.18.0 and then Octoprint to 1.5.3. Alle worked seamlessly. Ever since that update when I upload a new gcode file via drag and drop into a folder it gets uploaded twice: into the folder and in the main folder of Octopi.
Thak you for the swift replies. I will pay a lot more attention to the template in the future.
The DisplayLayerProgress plugin is also the "culprit" in my case.