'unexpected output' error during activation of ithemes security

I installed ithemes security (better_wp_security) on an existing WordPress blog that I had updated wordpress to 3.9 on. Also updated all other plugins.

Immediately on installation I received the following error message in the admin install plugin screen.

The plugin generated 2832 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

I was able to deactivate the plugin immediately.

I tried changing themes, deactivating a couple of less common plugins, but on each retry to activate the same error reoccurred.
So I deactivated the plugin each time. It is still deactivated.
Has anyone experienced this or have any ideas ?

Or must it wait until the plugin is updated.

https://wordpress.org/plugins/better-wp-security/