Cubase 9.5.40 update - How to Solve the 1603 Error (PC)
Mine was missing the file indicated in the error pop up.
So, if you just goto C:\ProgramData\Steinberg\Audio Export Post Process Scripts and see if OpenInCubase95_64.aepp is missing
If it is, just recreated a blank file and name it “OpenInCubase95_64.aepp” and ensure .aepp and not .txt file type of course.
That allowed to update to continue.
If it can find the cubase95.msi ( the main full install package ) the error can be avoided.
There simply is missing one or several files, and it tries to acces the old full installer to retrieve them.
If the full installer is still on the system, a repair install also works.
Consult the vendor’s support documentation for the registry keys used by the software. Reset these to default or delete them if appropriate before trying to install the software again. Error 1603 can completely stop you from installing a new software package, but only for as long as the root cause still exists.