Hi Tobias,
Does someone has a working 12c environment and can tell me what permissions are set in the directories (x:\oracle; x:\oracle\oraclehome; x:\oracle\oraclehome\12102 and x:\oracle\oraclehome\12102\database)
• <SID>adm and SAPService<SID> with full control
Our Environment
-Oracle 12c
-WS2012R2Std
-SAP ECC 6.0
I think I have encountered your error before when I upgrading our system from 11g to 12c last month. You're just missing a folder which is need to be created manually you can find which folder is missing by going to db13>job log look for the error log; after creating the folder add the <SID>adm and SAPService<SID> in security
I think I can help you more since we have the same environment 
Regards,
James Elvin