We recently upgraded our environment to vSphere 5.5. The environment is protected by Veeam Backup & Recovery 7.0 R2 which supports vSphere 5.5. Prior to the vSphere 5.5 upgrade, backups were working without issue. Sure enough, after the upgrade, backup jobs started failing. The error message logged in C:\ProgramData\Veeam\Backup\<Job_Name>\Agent.VddkHelper.log was:

VixDiskLibVim: Not licensed to use this function

You may see this same error with any other vSphere backup product as well. Most solutions tell you that you either do not have vStorage API’s licensed for the host, or that the user connecting to vCenter does not have Administrator permissions. Trouble was that this same configuration was working prior to the vSphere 5.5 upgrade. I confirmed that the ESXi hosts did indeed have Enterprise Plus licenses assigned and that “Storage APIs” was listed under licensed features for each host. I also confirmed that the vCenter user account the backup product uses had Administrator permissions assigned at the datacenter level in vCenter – the same as prior to the upgrade.

After opening a support case with Veeam and testing several things, I tried adding the Administrator permission for the vCenter user at the top vCenter level instead of the datacenter (one level down). Sure enough, backups started working. So it seems that your vCenter user needs this permission directly at the vCenter server level in vSphere 5.5