Hi,
This is very strange. In your browser, can you try to open URL:
http://<your SonarQube URL>/deploy/plugins/index.txt
It should list all available plugins on this server. If sonar-javascript-plugin-2.11.jar is listed but http://<your SonarQube URL>/deploy/plugins/javascript/sonar-javascript-plugin-2.11.jar returns 404 then there is an issue on server side.
Maybe try to have a look at your server logs. Also try to restart server, and have a look at <server install dir>\data\web\deploy\plugins content
++
Julien