Can't see shared calendar of coworker

We recently switched to Office 356. Among coworkers we want to share our calendars. I can see the calendar of every coworker except one and we have no clue why.

Here is what everyone of us did:

1. Open office 356 in the web browser and select Calendar
2. Click on Share
3. Type the email of the coworker, in this case mine, permission are the default values (can view all details)
4. hit send

I receive the email in Outlook and hit the blue "accept" button. After the spinner disappears it the button is "view calendar". Nothing happens if I click this button. The coworker does not appear in my shared calendars.

The strange thing is only this combination does not work. My coworker can see my calendar but I can't see hers. This whole procedure works with every other coworker. Only this combination does not work.

We are both using Macs if that is important. We use the Calendar App on Mac to see each others calendars but her calendar does not appear there. Does not appear in the Office 356 calendar web app either so I am not suprised.

Additionally I am the only one in the office with the locale set to US on my mac. Keyboard, language, etc. Everyone else is using German.