This error indicates that the cryptographic random bit generator could not be initialised.  This usually indicates that there is no good source of entropy available.

Some versions of the VMware hypervisor provide a "side channel mitigations" feature which unfortunately breaks the entropy source used by iPXE.  You can disable side channel mitigations using the Advanced Settings for the virtual machine.  See https://kb.vmware.com/s/article/79832 for more details, including the security implications of disabling side channel mitigations.

There is a known bug in the Microsoft Hyper-V hypervisor (as used for Microsoft Azure virtual machines), in which the emulated CMOS RTC hardware seems to be incapable of generating an interrupt.  This breaks the entropy generation mechanism used by iPXE, and will result in this error.  There are some indications that this problem affects only AMD hardware, though this has not been proved conclusively.  If you are using Microsoft Azure, then you may be able to work around the problem by using a larger instance (which is more likely to use an Intel CPU).