Maik Seyring We are experiencing the same issue since recently in Outlook Windows app. This post from SuperSimple365 from April 2021 talks about what sounds like the same feature with the addition of new daily or weekly digests, just being rolled out this year "will begin in late May and is expected to be complete by late June 2021 on Outlook Web App, Windows, Mac, and iOS and Android Outlook Apps." https://supersimple365.com/new-design-and-functionality-for-yammer-discovery-and-digest-emails/
The warning takes up half the screen so it makes the emails very hard to use. And I don't see a way to reply or react in these daily digest emails, which I've been getting for a long time now. I can 'open conversation' or 'show more' to open Yammer, but not interact directly in the email as described. I believe the error is telling me it can't be done. But why? Is my only option to turn off these digest emails? That would be not desirable since they are informative. I'd rather turn off this interactive feature, which seemed to be blocked for Yammer group emails coming from outside my organization.