Just another quick one today. I was faced with this error this morning when arriving at my desk and thought I would share the resolution.
Multiple sign in attempts failed, even after restarting the client. After coming across this kb article https://support.microsoft.com/en-us/help/2604176/you-can-t-sign-in-to-skype-for-business-online-because-the-certificate-can-t-be-acquired-or-validated I simply selected 'Delete my sign-in info'.
I was then able to sign in succesfully.
It appears there was cached user sign in credential corruption or possibly an issue or corruption with the certificate - deleting the sign in information resolved the issue.
Oliver Moazzezi – MVP Exchange Server