According to Google it could be any of:
1) Failing SATA-cable, try another one and see if that fixes your problems.
2) Failing harddrive, try replacing it (if you got a spare) and see if that solves it.
3) Failing SSD firmware, for example Intel 530 seems to have issues where an SSD firmware update might solve the issue.
1) Using mSATA drive in NUC, so no cable.
2) Possible, although I'm not seeing any failure indicators while running NUC's OS
3) Went through the firmware update process for the mSATA drive and it was already up to date.
Did a BIOS update of the NUC to 0033 and that seems to have resolved the issue. Not sure why it stopped working after more than a dozen successful PXE boots.
A wild guess if this problem returns, could it be related to https://qdms.intel.com/dm/d.aspx/AC9C0719-A458-4D4C-A9DE-F7F24375D169/PCN113335-00.pdf ?
Description of Change to the Customer:
A resistor value on the SATA Hard Disk Drive Daughter Card will be changed from 2K ohms to 1.5K ohms to increase drive strength.
Im thinking if your issue (even if its not directly related to above) could be related to PXE failing after a couple of successful boots. Something to test if this reappears is if this "counter" of successful boots resets if you make your NUC completely powerless (unplugg the PSU and wait for a minute or so before reconnecting it) compared to if its just rebooted.
That is if you make your NUC completely powerless the PXE boot will work but not if you just reboot the NUC.