Viber suddenly stopped delivering my messages

Hi, Viber has suddenly stopped sending/delivering my messages to only one contact. I could until maybe a week ago use viber as normal with this contact. Once I sent a message I would get the normal sent/delivered updates and my friend would receive my message. This has suddenly stopped happening my friend is no longer receiving the messages I am sending. At my end, I send a message and it sits in the chat box with no sent, delivered, or error update of any kind. It simply sits as a message cloud that my contact never receives. I can still receive their messages as normal. I tried waiting and trying on different days and in different wifi circumstances but this made no difference. Other contacts can still receive my messages, it is only this one. Updating to the latest version also made no difference. I changed no settings immediately before or after this starting happening. Can you please help tell me what is happening and what I can do to fix it? Is it likely a probably at their end or my end? I am using viber in Japan on a data + viber plan with Rakuten. Viber was operating normally on this place before this problem occurred. Thank you! Chris