I was experimenting with dashboard elements and global context on NodeRed 3.0.1. I was using the injector node buttons to simulate various actions. At some point all the injector buttons (across this and all other flows) have been greyed out. All the debug nodes are also grey (and nothing is showing in the debug panel).
I have noticed that the node-red UI doesn't always notice when the backend goes offline. It is good that the elements on the web page don't cause the whole page to reload when they change, but sometimes I make several changes and it doesn't seem to work quite right. Then pushing F5 to reload the browser shoes a Bad Gateway or not found error.
In this case, however there is an additional factor - Docker with Visual Studio Code.
When I run docker docker-compose up -d, VS Code pops up a message asking if I want the NodeRed port forwarding to my laptop, where I can connect via localhost. In this case I restarted docker-compose and did not use the link proposed by VS Code. I carried on using the old web page (i.e. I did not reload the page).