Have a look at the cloudbackup\operational event log on the client. verify the logs to find the cause, logs are in C:\program files\Microsoft Azure Recovery Services Agent\Temp directory for CBEngineCurr.errlog for detailed information.
Have you get a chance to see this thread,
https://social.technet.microsoft.com/Forums/en-US/3ce0fdf5-f77e-47e9-8a56-3c85e3324a54/verifying-backup?forum=windowsazureonlinebackup
Regards, Rajukb | MCSE (Communication ), MCSA (o365) ,Certified "Lync server 2013 depth support engineer"| This posting is providedwith no warranties and confers no rights. If my reply answers your question please mark as answer/helpful if its
helpful.