Unknown status in Outlook 2016 for external users

Hello,

For many days, I try to find why I cannot see external users status in Outlook 2016. It runs perfectly with tenant users, but external users show a white circle with an unknown presence label. And I cannot start an IM, no IM window opens when I click on the icon.

It runs OK in Teams client and Outlook web.

Many thanks for your ideas and help.

Best Regards