Hello again,
sorry to bother you, but some different searches with operators in the specified dorico forum did not yield any results. (Is the IT-compartment of Steinberg really aware of the problems with the forum search?)
I tried to zoom in on Win7/64 with ctrl + (both possibilities). Nothing happened. Is this a bug? Does it work on Windows 10?
On Windows 10, German Keyboard, there two shortcuts to zoom in:
y (x to zoom out)
Strg + ´ (Strg + ß (to zoom out)
Hope that helps.
Thank you very much! That works in Win7/64 as well - when I switch to german in dorico 
Two comments:
I like to work in the english language with notation software since I learned SCORE nearly thirty years ago which was never available in german. The combination english in dorico and german in windows with german keyboard does not work with either key commands.
In nearly every windows software the key command ctrl (Strg) + + and ctrl + - is used to zoom in and out. IMO, it should be the standard assignment in dorico as well. So far as I could determine it, this command is not used for anything different yet (I tried with the combination english (dorico) with german (keyboard) and german (dorico) with german (keyboard)).
If you’re running Dorico in English but with a German keyboard, you should still choose German language in the Key Commands page of Preferences, as the English shortcuts simply won’t work on a German keyboard.