Ever since I have changed to guessVhdSizeOnImport: true I have not had any problems. However I only backup 5 VMs which is not much.
In the past I noticed that if I didn't change the default "Concurrency" setting to 1 in the Backup NGs advanced settings (I assume that it tries to parallelize backups by default) then my tasks would fail randomly once in a while with VDI_IO_ERROR(Device I/O errors). I figured that perhaps my servers are not fast enough; they have about 40MB/s write speed to the disks only. Setting to 1 solved it, but I can imagine this may not be desirable if there is lots of VMs to backup since it would slow down the process considerably.