Why do I get the message "There was a sync problem"when I try to connect my master site to

I installed the plugin user sync on my 2 sites I choose which one is the main and the sub, I got the Main URL and the key that I put into my subsite but when I click on connect this site to the master I got the message"There was a sync problem." And there are only 13 users synchronized but with no roles, no informations.

Could you help me?

Thanks.

Thibault