Hi Stirling,
It was only yesterday that I could find the time to try out your solution.
I set up my iPhone as a new device, then I added my Gmail to the accounts (including agenda).
Unfortunately the Agenda app still did not read the agenda items of my girlfriend who's shared her agenda with me.
When I installed the Google Calendar app, it read those items right away.
Also on my Macbook Agenda app they are visible.
Since this error occurs on a "new device" , I imagine it is one of those things that nobody can really solve...
Anyone familiar with this issue?
For me the work-around with the Google Calendar to check my girlfriends agenda is okay.
Just not how it should be ! 🙂
Thanks anyway for the support.