OK, this might get a bit technical and is more suitable for the Kodi developers among us:
Streaming from .strm files just doesn't work anymore in Kodi,. After some investigation I discovered that the stream URL's redirect a couple of times and that the final URL, with a long one-time generated...
I can confirm, streaming from strm files works again. I hope it stays like this. Last week Friday is the last time I saw it working for about an hour. I have no clue why the streams didn't work, maybe it took too long for Kodi to startup and at gave a time-out.