On the groov box, log into https://IPorURL:3000 and click on the link on the left.
On Windows, its in the 'DOS/CMD' terminal that you open to run Node-RED.
Also, you can get some debug info from the debug tab in Node-RED its self, but I am guessing you have already looked at that and not got enough info.
Sometimes placing a 'Catch' node anywhere in the flow with its output to a debug node with the full message option selected. (But that is mostly for crashes rather than just disconnects - but thought I should mention it).