Upgrading an existing Android installation to a major new Android version seems to always give mixed results. And it seems with your Fairphone 2 you have gone through that twice, from the original Android 5 to Android 6 some time ago, and now from Android 6 to Android 7.
In case the suggested factory reset doesn’t do the trick (meaning your problems would not be related to Apps or user data), I would recommend a real OS cleanup …