Build status notification fails with "Enter a valid URL"

I've setup my environment as described in this video: https://www.youtube.com/watch?v=uu5XcU4EPzQ

However, when build starts and finishes, I don't see the status on BitBucket commits. In Jenkins log I can see following messages:

May 13 10:57:18 rambo jenkins[743]: INFO: Bitbucket notify on start
May 13 10:57:19 rambo jenkins[743]: May 13, 2016 10:57:19 AM org.jenkinsci.plugins.bitbucket.BitbucketBuildStatusNotifier notifyBuildStatus
May 13 10:57:19 rambo jenkins[743]: INFO: This response was received:{"error": {"fields":

, "message": "Bad request"}}
May 13 10:57:19 rambo jenkins[743]: May 13, 2016 10:57:19 AM org.jenkinsci.plugins.bitbucket.BitbucketBuildStatusNotifier prebuild
May 13 10:57:19 rambo jenkins[743]: INFO: Bitbucket notify on start succeeded

Any idea what has been configured wrong?

Note: our Jenkins server is inside our local network and generally it is not available from public web. The exception is hole opened exclusively for BitBucket's IP ranges so that push notifications can be sent to Jenkins from BitBucket's webhooks.