Hi.
Just to make it round: We updated the Lync2013 environment with the latest CU. After that we deleted the SfB SBA with the Topology Builder and recreated a new SfB SBA, reinstalled the SfB 2015 SBA on a new WinServer 2012 R2 (new hostname) and installed the
SBA role on it.
After that all works fine.
Regards
Frank