MarkDown Apps not available

Great idea in order to let users know the purpose of a folder.

Unfortunately, I cannot get this app working.

• Installed the app
• Markdown app is also installed and working
• Created "README.md" in some folder

Result: No readme is showing up when browsing to that folder. I only get an error in the browser's console (tested on Firefox and Chromium):

Do I miss something?

Edit: Forgot to mention that I'm using Nextcloud 15.0.2.