I have a Galaxy S9 and only recently I have found an issue with group messages.
When I enter the group chat screen, the text box is faded and un-clickable with the message "Unable to send messages". I am not able to interact with any of the buttons on that element. (The '+', the text box, the sticky, nor the voice)
If I get a message to a group chat, I am able to reply in the quick reply section of the notification without issue.
I am able to start a "new" conversation with all of the participants. When I create the "new" conversation, the old chat history is brought up and I can read previous messages and then send my own. However, after I send the one message, I am again presented with the "Unable to send messages" error. The message I sent from the "new" conversation does go through and I can see it as well. (This isn't ideal for chats with many people. Not really ideal at all, but it is a work-around.)
Individual chats are working just fine. I haven't found any issues.
Restarting the device (several times)
Toggling Airplane mode several (several times)
I have tried leaving the chat and then starting a new one. However, all that did was mark the chat as 'inactive' in the list and still present me with the same issue. (Is there a way to reactivate a chat?)
I deleted the chat entirely and started it up again and the issue seems to have been resolved. Though I would prefer not to have to do this with all of my group chats.