Simple Calendar Google Api Issue

“While trying to retrieve events, Google returned an error:

Please ensure that both your Google Calendar ID and API Key are valid and that the Google Calendar you want to display is public.

Only you can see this notice.”

This is what I read on my page. I did it three times with help of the instructions. No chance to get it running. The Google Api Key tut seems to be an older one. Did anything change recently in setting up a google api key for simple calendar plug in? how to handle key limitations of my google api key? which setting should I chose? no limitations? webserver?

my calendar is public.
I use the right calendar id. i use one calendar out of many from my calendar account.

wp 4.6.1

please help? instructions and tuts did not help sufficently…

best, timo