The plugin generated 6 characters of …

Hi,
I’m using this plugin for my client, but i’m having some problems 🙂 It worked fine, I made matches, seasons, etc., but once I created a post and wanted to load up a featured image to it and nothing happens. So I did the usually steps: deactive all plugins first and trying, trying, trying. If I’am activing the legaguemanager plugin, I got this message in the admin:
“The plugin generated 6 characters of unexpected output during activation.”
The plugin works fine, but since that I can’t upload featured image, in Firebug Console I see this when trying to upload image:

http://postimg.org/image/gl6o79r8n/09b6a701/

If I’am deactiviting the plugin, the featured image works fine. I made a backup from db, reinstalled wp, restored db, installed the plugin, it worked fine, the featured image also. But when I started to work with the plugin, after a time I got the same problem: can’t upload featured image, in the Firebug Console I see the attached error. Tried with Chrome and IE, same problem.
When I first install the plugin, I don’t get this error, so probably the error comes out after some working with the plugin, but I don’t know when or where.
Some said it is whitespace problem, I’ve searched all the php files, but nothing 🙁
Can somebody help me?
P.S. Sorry for my english 🙂

http://ww.wp.xz.cn/plugins/leaguemanager/