I didn’t know that WhatsApp had a local backup feature. I lost all of my data but it wasn’t anything important. However, the problem returned sometime after I added a second number. I reinstalled the app and set up only one number. It’s been working fine since last night.
While I was writing this, I noticed that WhatsApp rolled out an update. I installed it, added the second number and haven’t run into any issues yet. I’ll post another update on its status later.