I have the same issue on my Lumia 950 XL. Lately WhatsApp beta just gets stuck on the loading screen randomly, and it doesn't open for hours. Then randomly gets back to work many hours later.
I wrote to WA via Android app, but I fear they will tell it's a Microsoft issue and we all know Microsoft left this Windows 10 Mobile OS to die.
I'm still using my 950 XL because of its great camera, audio/video recording and stylish design with Mozo leather cover, but now it's a real problem if WA doesn't work and I have to use a 2nd phone to use the app.
I wonder why this happens because I never had this issue in 3 years, just recently.
The funny thing is:
I wrote to WhatsApp customer service from my rooted Android Phone and they don't want to give me an answer on my Lumia, because they don't even bother to read my email since it's sent from a rooted Android phone. This is hilarious and ridiculous.
Human stupidity at its best. How the hell am I supposed to contact them if WA doesn't work on my Lumia?