Yes, there seems to be a big issue in build 14977 for non-UWP apps like WhatsApp, basically they no longer work. All the WP8 apps I wrote or reinstalled after my upgrade to WM10 (which have always run perfectly on WM10) now just get stuck at the loading screen showing a clock icon or 'loading...'. And when they are in loading the state the battery drain shoots up - I left my custom weather app on the loading screen for about 15 mins to see if it would actually start and it used about a quarter of my battery - no exaggeration. I'm on a 950XL.
Strangely this has only just started happening for me in the last couple of days despite 14977.100 being installed over a week ago.... how can this suddenly be happening?