If users have a Yammer account, but can't log in to on their device, they may need to update Android WebView.
If the above step didn't work, look into these potential issues, or contact your IT administrator or Yammer Support:
An external certificate that is not yet valid
An external certificate that has expired
An external certificate that doesn't have a private key
Note: These three issues are related to the Web Application Proxy (WAP). For more information about WAP, see Web Application Proxy.