Opening a .apk in Android shows a file list instead of installing

For a client I have build a private app, just a few devices use it. The user can download the .apk by navigating to a certain URL on their device. A few weeks ago everything was working great, but since a week the .apk shows a file list instead of installing the app.

Does anyone has any idea wat the cause of this problem may be?

The customer is opening the .apk file in the wrong app - it should be opening the default installer (whichever one the particular device has set as default), not a .zip extractor (which is what appears to be happening - since an .apk file is just a .zip file with a different extension).