Wordpress User Sync stopped syncing

unfortunately User Sync just stopped working. I disconnected the sites, deinstalled the plugin options, connected the Master and Slave sites again in Debug mode and the message in the logfile is:.... "checking key false".
I see the Subsite in User Sync page on the Master site. When I hot the button to sync, it gives me a message that the Sync was successful but there is no date saved in the Last Synced date on the master site.

The two sites are installed as following:

Master site: http://site.com/1
Sub-Site: http://site.com/2

When, I installed the plugin for a first time couple of days ago, all worked perfect. It connected the two sites. It was transferring the new registered users on the Master to the Slave, syncing existing ones, etc., and then suddenly it stopped working.

I need help ASAP. We are going live in the next days and User Sync not working is a big stone on the road for us.

Looking forward to hearing from you.