When using the VMware plugin for backup jobs on smart clients, this error may happen when the SSL certificate is changed:
Job Message 2018/08/23 21:09:34 970 Data Plugin vc001.com.local [21.com.local] Writing disk data to media
Job Message 2018/08/23 21:09:34 970 Data Plugin vc001.com.local [21.com.local] Backing up disk '[001.000.002] 21.com.local/21.com.local.vmdk'
Error 2018/08/23 21:09:34 970 Data Plugin vc001.com.local [21.com.local] Failed to open virtual disk
Error 2018/08/23 21:09:34 970 Data Plugin 001.com.local [21.com.local] Failed to backup data of disk '[001.000.002] 21.com.local/21.com.local.vmdk' to media