I found a workaround. I hit F1 and Cubase Pro 9 told me it couldn’t find the file “Operation Manual” in (windows) Program Files\Steinberg\documentation
I created a new directory at that location and copied my PDF version of the Pro 9 manual there. Now, when I hit F1 the Operation Manual opens. 
Please note, that none of help documentation was transferred during my update. All the old files were still in the Cubase Pro 8 folder.