As you may be aware, Microsoft has introduce a new security/identity feature on Yammer to enforce all users to use their Office 365 credentials.
It may be one step to get closer for the long waited unified directory synchronization tool for both Office 365 and Yammer, but this is the subject.
If you enable this new feature – Enforce Office 365 identity in Yammer – and are using Yammer Directory Synchronization tool, the synchronization may/will stop working soon after (and you should be notified with the following email)
The most recent active directory synchronization attempt failed. The following error(s) occurred:
Unable to authenticate to Yammer.
This could be because you have set on your Azure AD a User Password Reset Policy which requires them to register at least one secondary authentication method to allow them to reset their password.
Doing so, and because you may have use a service account to configure the Yammer Directory Sync, this settings will then automatically applied to this account, which obviously may not be registered for self service password reset.
If you got such notification email soon after activating the Enforce Office 365 Identity in Yammer setting, try to logon to Yammer using this service account and you should get the prompt to register for self service password. Just register the account for self service password reset and you should solve the issue.
NOTE you may have to follow the following KB https://support.microsoft.com/en-us/kb/3015691 when you will try to re enable Yammer DirSync