pro sites conflict with another plugin! help!

Hi,
i have install new plugin in my multisite environment.
the plugin name is "VoucherPress"
http://wordpress.org/plugins/voucherpress/

after i have install the plugin and tried to make some tests i start getting empty pdf files that gives error.

then i look at the support form, and the author suggest to disable other plugins to see with one make this problem.

so after some time i found that the "pro sites" plugin make this problem!

from what i see the author of the plugin write:
This is almost always because another plugin has sent some content (even just whitespace) to the browser before the voucher has been created.

so guys need your help to fix this problem so i can work with voucherpress plugin.

if you need to test this, i can send you all the login details so you can see the problem.

Thanks