The Book

https://www.veeam.com/kb1909

In Veeam Backup & Replication, if more than 100 jobs are running simultaneously, and the user attempts to start more jobs, they may fail to start with the following error message:

Error Job <JobName> cannot be started. SessionId: <UUID>, Timeout: [361 sec]

A Veeam Agent for Microsoft Windows backup job may fail with:

Managed session <UUID> has failed 

In the Veeam Agent for Microsoft Windows service logs (Svc.VeeamEndpointBackup.log) following errors are found:

Error Job <JobName> cannot be started. SessionId: <UUID>, Timeout: [361 sec]
Error Job [Infrastructure update] has failed to start. Timeout expired (6 minutes). SessionId: <UUID>

The following event is found in the System event logs:

Log Name:      System
Source:        Win32k
Event ID:      243
Task Category: None
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:     
Description:
A desktop heap allocation failed.

By default, Veeam services are run as the LocalSystem account. As a result, all processes are started under NT AUTHORITY\SYSTEM context. If the Windows OS is running out of Desktop Heap memory, new processes can't be started.