“headers already sent” problem in WP4.0.1

I’m getting this error on activating the plugin:

The plugin generated 1 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

The plugin was working ok on the old site (WP 3.8.5 and PHP version 5.3.24)

When I made a clone (same WP version) of the old site to a new server (PHP 5.3.29) this plugin caused the site to stop working so I deactivated it.

Now I’ve updated all other plugins and WP to v4.0.1
Activating the plugin again is giving the above mentioned error.
So my conclusion is this plugin is not working in WP 4.0.1.