Normaly under the the Point "SQL" in the Exchange Processing Settings there is "Do not truncate logs" choosen. You must set it to "Truncate Logs"
I know it has normaly nothing to do with the Exchange Log File Backup which you can set under "General" . But it will do the Trick for the Bug to set the SQL Logfile Truncate for the Exchange Logfile Backup
Before , means no Logfile Backup from Exchange possible ( Truncate [FALSE] )
After doing the Trick , now Logfile Backup works for Exchange Server Logs ( Truncate [TRUE] )
I hope this helps for other Customers to solve the Bug .