How to fix "no recipient" glitch

I have a galaxy S6 and recently a strange error has popped up. I can't find reference to it anywhere else:

So, my fiancee's incoming texts (texts sent to me by her) are coming in under a different tab than my outgoing texts (texts sent to me by her).

The incoming text tab is usually titled "New Message" unless I have unread texts. In that case it shows as the same name or number as the last sender.

If I try to reply or change settings on the incoming texts tab, I get the message "No recipient." I have deleted both conversation tabs, and it continues.

Additionally, if I send a picture, it will appear in the "new message"/incoming tab, but it will be marked in the outgoing as "sending." It will always say sending, and will jump to the bottom of the que. It can't be deleted from the outgoing except by deleting the entire conversation.

This only happens for her messages. She has a galaxy s7. She does not have any problems receiving my messages.

It's been going on for about a month.

Um... I think that is everything I know. If anybody knows anything about this, I would be grateful.