Nice error with the Exchange 2013 installation (I think sandbox assembly sounds kind of funny):
[03.26.2015 17:17:52.0050] [0] [ERROR] An attempt was made to load an assembly from a network address, which in previous versions of .NET Framework would have resulted in the assembly being executed as a sandbox assembly. In this version of .NET Framework, the CAS policy is not enabled by default, so this load operation can be dangerous. If you do not intend to create a sandbox assembly by this load operation, enable the loadFromRemoteSources switch. Further information can be found at "http://go.microsoft.com/fwlink/?LinkId=155569".
[03.26.2015 17:17:52.0206] [0] Finished loading screen InitializingSetupPage.
[03.26.2015 17:20:10.0034] [0] The registry key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ExchangeServer\V15\Setup" was not found.
[03.26.2015 17:20:10.0034] [0] CurrentResult setupbase.maincore:396: 1
[03.26.2015 17:20:10.0034] [0] End of Setup
It's always nice when error messages are translated into German 
By the way, the solution is not to install Exchange 2013 from a network drive.
Hm, leider ist das bei mir nicht die Lösung. :/
Ich führe Exchange 2013 CU9 Setup.exe aus einem lokalen Verzeichnis auf C: aus und bekomme trotzdem diesen Fehler. Hast Du da noch eine Ahnung?
Ich vermute bald, es liegt daran, dass ich in einem Standort unserer Domain installiere. PrepSchema und PrepAD habe ich bereits erfolgreich am Standort des Schemamasters durchführen können. Dort läuft auch unserer bisheriges Exch2010. Nun will ich das erste 2013 aber in einem anderen Standort installieren, was zu obigem Fehler führt.