User Sync doesn't works?

Hi,
I've tried to uninstall all the plugin, change the role of master and sub site hundreds of times. And it doesn't works.
The texts in errors_s.log:
[2013-05-28 10:25:38][S] 04 - sending request: something went wrong**?The texts in errors_m.log:
[2013-05-28 14:07:35][M] 02 - sending request - url=/wp-admin/admin-ajax.php?action=user_sync_api;;***
[2013-05-28 14:07:35][M] 04 - sending request: something went wrong***
[2013-05-28 14:07:35][M] 06 - checking key false;;***

There must be something wrong. Could you help me to figure this out?