I found the parameter in Avast to solve my problem (see below).
I have desactivated in the “Enable Hardened mode” options.
I have created an exclusion for the Cubase path.
Ihave reactivated the Hardened mode.
I sent a request concerning a problem to install Cubase 8.5 and a error message like “don’t find the path”.
I try to stop my antivirus. During this period all work correctly until the antivirus is been reactivated.
I can work fully with cubase 8.5 but without Avast.