I encountered the same problem. There was "Request 4 cancelled on connection closed" error and will not fix itself after a reboot. There is no info in Debug Console or Output.
I tried to run the Arduino IDE from console, no helpful output either.
I tried to move the sketch to another place and it fix the problem.
Update: The bug is mentioned in Unable to start debugger when watch expressions are configured · Issue #1582 · arduino/arduino-ide · GitHub