Bug 28229 is being investigated and will be addressed in a future version of NVBU.
If your devices remain offline after a NVBU restart, it is important to verify or exclude that the issue is not hardwar related. Sending NVBU logs in binary format to Support will help establish this.
If the issue is not hardware related, ror the time being, users will always have to verify the status of all devices after a NVBU restart of the machine controlling these devices.
Use the CHECK button to bring it back online via the Web Interface, or the nvcheckdrive cli tool on command line.