FSSO polling mode problem

The Collector Agent can communicate with the AD.

If I press "Show Monitored DCs" at the Agent GUI I can see the number of "# Logon Events" increasing all the time. 

If I press "Show Logon Users" at the Agent GUI the list is empty.

The Only FSSO info available at the Fortigate is the directory structure of the AD i.e. the available groups.

It seems to me that the access of the Collector Agent should be OK since it is obviously detecting login events.

What would be the first few checks to troubleshoot this problem.