A failed quiesce snapshot will not make the backup fail. However, the backup might not be application-consistent.
Quiescent snapshots are entirely managed by the ESX/ESXi host. It may happen that one of these snapshot fails due to several possible reasons.
In order to better understand what is causing the issue:
2.Using vSphereClient select the Virtual Machine and perform a quiesce snapshot called "Test Snapshot with quiesce" (quiesce enabled, memory disabled)
Once the snapshot is terminated, download ESX log file using VM Explorer:
1.Open VM Explorer
2.Switch to Datacenter tab
3.Select the ESX server where the Virtual Machine belongs
4.Once the server is refreshed right-click on the ESX server and choose "View ESX Log"
5.Save the ESX log
The ESX log might contain clues about what causes the snapshot failure