Failed to set Build on <hostname>: execution expired

Sometimes provisioning from PXE doesn't properly start, anaconda just
hangs. After resetting the machine, the OS installation starts, but the
host is left in build mode due to the following error:
2016-04-27 00:28:10 [W] Failed to set Build on <hostname>: execution expired
2016-04-27 00:28:10 [I] Completed 409 Conflict in 12169ms (ActiveRecord:
12.3ms)
I'm working with compute resource VMWARE, interacting with foreman using
REST API.
Can you give me some pointers to better understand the behavior?
Thanks