An issue was reported this morning where new messages could not be sent for long private message conversations where the number of messages exceeded 20. Conversations with more than 20 message triggers pagination, but unfortunately at the moment, any new message submitted in the form on the second page fails to have any effect, and no error message is shown.
The issue is being looked into right now. In the meantime, the workaround is to start a new conversation. Apologies for the inconvenience caused!
The bugfix has been rolled out.