I just ran into the same problem on Meta when I had the composer open in two tabs. Attempting to reply to the topic, the popup from your screenshot was displayed. That popup window is expected to be displayed, but underneath the heading, the original topic’s title should be displayed. It seems that Discourse is not finding the original topic.
When I clicked the Reply on Original Topic option, the following error was displayed:
This seems like a bug. Unless someone knows what is causing the problem, we’ll have to work out the steps to reproduce the issue. I’ll get a chance to do that today.