User Sync doesn't works?

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?