Debugger won't start (but Run will)

Hello,

I'm a newbie, so go easy on me!

I can't get the Debugger to work.
"Run" works just fine.
But when I use Debugger (with any project), it opens the game in a Firefox tab like normal, but it's just a blank/black screen. (It's not exactly completely black, but a series of dark grey concentric ellipses - the normal way it looks right before the game starts up.)

Sometimes I get the "Unable to connect on port XXXX" error, but this seems random and only shows up some of the times.
I've...

Debugger won't start (but Run will)