Thanks to Patrik Kernstock, who works in our Technical Support organization at VMware, for making me aware of an issue related to Nested ESXi running on an ESXi host that has been upgraded to ESXi 7.0. Several folks in the community have noticed after upgrading their Intel NUC 7th Gen and deploying a Nested ESXi VM and powering on an inner-guestOS would causes the Nested ESXi VM to crash.
Upon further investigation, it looks like this is not specific to the Intel NUC platform but rather with a specific generation of CPUs which are Intel Sky Lake-based and as a result, some customers are noticing this affect on their 7th Gen NUC.
UPDATE (06/23/20) - ESXi 7.0b has just been released and contains the fix for the Nested ESXi VM crash. If you are using an Intel NUC 10, do not just apply the patch as the updated ne1000 VIB within the patch will override existing Intel NIC driver causing the network adapter to no longer function. It is recommended that you download the patch and replace the default ne1000 VIB using Image Builder with the Intel NIC version before applying the update. To download the patch, please visit VMware Patch Portal site.
The good news is that this issue has already been reported and we should have a fix in a future update of ESXi. In the meantime, you can still run Nested ESXi and Nested Virtualization on these affected CPUs, you just will not be able to power on inner-guest VMs. Big thanks to Patrik for helping out with the testing and triaging this internally.