The specified transports are not available

The SAN or Virtual Appliance mode intermittently fails with the error:
 

Error: Client error: The specified transports are not available. Available transports: [nbd]. Specified transports: [san]. VMDK path: [[Datastore] VMname/VMName.vmdk].

The following entries can be seen in the VeeamAgent log:
 

diskLibPlugin: [timestamp info 'transport'] Connecting to host symcVC.symantec.com on port 443 using protocol https

diskLibPlugin: [timestamp trivia 'transport'] Initializing SSL context

diskLibPlugin: [timestamp trivia 'DiskLibPlugin'] Setting error in state 1 : class Vmacore::SystemException(A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. )

diskLibPlugin: [timestamp error 'DiskLibPlugin'] Cannot use advanced transport modes for vc/moref=vm-xxxx/snapshot-xxxx: Other error encountered: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. .

 diskLibPlugin: [timestamp info 'DiskLibPlugin'] Successfully released all resources.

 VixDiskLib:  Advanced transport modes not available for opening moref=vm-xxxx.

VDDK library is not able to utilize advanced backup modes - SAN or Virtual Appliance.

The issue is caused by an unreliable network connection between the Veeam Backup & Replication server and the vCenter Server.