This topic for everyone who has problem with sync when activated sub sites.
I will try to help you.
What I did:
I added feature for writing all actions (step by step) in plugin's log file. It's help me understand on which step you had problem.
What are you must do:
1. Select checkbox "Use Debug Mode" on main plugin page before select "Master" or "Subsite".
2. You should activated Debug Mode for both sites - Master and Subsite.
3. Try connect Subsite to Master site.
5. Copy log files /plugins/user-sync/log/error.log from Master site and Sub site, then rename them to "sitedomain.log"
6. Send links for download these files from the contact form on page https://premium.wpmudev.org/contact/ - with mark in subject [for Andrey]
7. Wait my response.
ATTENTION: Please, don't attached these files or links on them in this topic, these files may contain PRIVATE information!!!