Cannot share calendar with external person in Outlook for Mac

Hi,

In a company with 2 different M365 tenants, I tried to share a calendar between 2 people from both tenants but without succes and with the following message:

"Outlook cannot open the folder, U don't have the permission to open this folder. Contact (user mailaddress) for further info"

What did I allready performed:

• Under admin settings/organisation settings, I've checked the sharing settings for calendar, all fine.
• Under exchange admin/ organizaton, I activated the 'organization relationship' on both tenants for each other.
• On both Mac's I added the external user in the permissions to share the calendar  with the external user.

What am I doing wrong?

Regards,

Froota