I'm experiencing more or less the same problem the difference is that mine doesn't wanna reboot it says boot device is missing!
This message is typically (almost exclusively) what you see when the BIOS cannot successfully initialize the memory.
How long have you had this WY NUC? Over time, as components wear in the processor, on the DIMMs and on the motherboard, additional noise can be produced. This can eventually reach thresholds where data on the memory buses becomes garbled.
What is/are the speed of the DIMM(s)? The faster the speed, the higher the chance of noise-induced failures.
If two DIMMs, are they matched (i.e. same size, wait states, etc.)?
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Please let us know more information regarding the memory, just like mentioned before this could be related to the RAM.
I would also recommend a BIOS recovery to the latest version, find the instructions and file below;
Please let me know how it goes.