Release Notes

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Incremental Crawl: The Yammer Connector supports ChangeSet functionality for Messages. All messages newer than the latest saved message, are retained. 

DocumentHandler

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Multiple connections can be configured, with similar or different content targets and Security options.

Conditional Access:

Make sure that you don't have any conditional access rules which disable access to Office365 applications. Check if there is no restrictions for index user's location, device etc...
Also in 'Grand' tab under Access Control "MFA" shouldn't be checked, because connector doesn't support it.

For more info see: https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/howto-conditional-access-policy-block-access