But beside that error, some apps, at least Spreed.ME, are in the store, showing updates, but still not beeing able to update, throwing the SSL error while trying.
For Spreed.ME @MorrisJobke writes on GitHub:
Sorry for this back and forth, but this is actually an issue with the environment where the archive of this app is hosted. The SSL cert of this server is not fully trusted and we can do very little there. Sorry for the inconvenience. The app author needs to place the app on a properly hosted platform.