Check your AD Sync group configuration and make sure the following options is set:
Systems that exists elsewhere in the System Tree: **Set to --> Move systems from their current System Tree location to the synchronized group
Thank you Jeremy
The option was already set to 'Move systems...' but none of the Security Group Members are sorted in the ePO Tree Group when clicking 'Synchronize now'. The 'Members'-Tab of the Secuity-Group Properties in Active Directory users and Computers lists about 2000 members.