You have extra space character in your addon id. Maybe that's the problem (well, not maybe but very probably).
And you can try to disable from command line. And you will also get some feedback.
Code
curl -d '{"jsonrpc":"2.0","method":"Addons.SetAddonEnabled","params":{"addonid":"plugin.video.demoaddon","enabled":true},"id":1}' -H 'Content-Type: application/json' http://localhost:8080/jsonrpc
I'm using such code but it is at home. Try it and I will double check later today.
Update: Actually I found it on Dropbox too. And it is the same as above. And it works fine.