I'm having Hangout problems. Some contacts that have history texts to different numbers (for different countries) and sent by me from different numbers (from different countries) have been getting this error...
After a few back and forths of deleting the app, reinstalling it, unlinking Hangouts from the voice app and re linking and merging conversations it then works for a couple days. And if I don't merge conversation btw, it creates a new conversation for every time I switch (the sending or receiving) numbers within a contact.
I know, I know, just stick to WhatsApp.