I had the similar error last week like mac_kissack. All the contacts have disappeared. Messages werent sent out nor i can receive it. Initially i have deleted the ~/.wazapp folder , uninstall and reinstall the program. It works for a while, then it's back to the same problem. After a number of retries, i give up. i chose to reset the settings. It works...
but for 2-3 days only, then it has the same error again.

I'm not sure which bug causes this..then i resort to restore my old backup. Surprisingly, most of my apps, documents, videos, pictures are preserved. Just some of my contacts , accounts and some settings are gone. If you wanna do that, it's best that you do a backup first. Anyway , my whatsapp is working till now..(0.9.19). Since version 0.9.19 is working fine for me , then i'll just leave it...

If this hasn't solve any of my problems, i will probably flash my phone and start from fresh. I'm contented now..hopefully this problem never appear again . Hope it helps...