Weird behavior with @mentions

Hello all,

we've experienced a weird behavior of Outlook (Microsoft 365), when using the @mention. If my colleague mentions a certain contact Outlook crashes(the same happens, when he adds the contact as recipient) . All others work fine. After trying some fixes, we removed Outlook und installed it new.  Now he can add the contact as recipient, but the contact won't show up under the @mention feature.

The problem is, that this certain contact is his main contact for a project. 

Does anynone have an idea, what the cause could be or how to solve this?

Any help will be appreciated.

Cheers,

Erik