I have a new Note 9 and this past weekend my two most used contacts will not allow me to reply to their text messages. The message in message + says that I am no longer a participant in this conversation. I still recieve messages in the same conversation but am unable to reply.
The same thing happened to me. I called vzw and a rep advised me to go to settings, apps and messaging + app. Then I cleared the data and restarted my phone. Once it powered up I updated all apps in play store. Once I reopened the messaging + app it worked fine. Hope this helps!
Having all the details before you complete an app reset is very important, jb3030. Your text message details are stored on the device memory, so resetting the Message Plus app like SWEETANGIEK suggested will not erase or remove any text messages. AdamB_VZW