KB1788: Credentials Test or Job Fails when attempt to use VIX
https://www.veeam.com/kb178808.07.2013 · Ensure the correct account is used. To use VIX for Guest Processing, one of the two following accounts must be specified for Guest Processing: The Built-in Administrator (i.e., hostname\Administrator) Note: This must be the original Built-in Administrator who has a SID that ends in -500.This user is unique and has the ability to bypass Windows User Account Controls.
KB3225: Troubleshooting Guest Processing "Test Now" issues
https://www.veeam.com/kb322523.07.2020 · Regarding Warning in VMware Environments. In VMware environments, Veeam Backup & Replication can use two methods to connect to a guest: RPC or VIX. If RPC is testing successfully, it is generally acceptable for the VIX test to fail as it will not likely be used.However, if it is desired to have VIX succeed, please see the relevant section at the bottom of the …