i have Lumia 630, and i have a problem that whenever i press back key to go the main list of chats from a private chat, it crashes. it was not there until yesterday and i have no idea what caused it. please help me.
It's pretty much a RAM usage issue. I once got that problem on my 520, which has 512 MB of RAM, just as your 630.
So, restart your phone, open WhatsApp, leave it for at least 30 seconds, let the messages be up to date, then start using. Using WhatsApp immediately after opening it causes it to crash sometimes.