Hello,
i found a following bug with my DN2820FYKH NUC i have just bought:
After changing the UEFI boot to legacy mode, the HDMI output will shutdown, until the OS loads. This is particularly dangerous, is you have no OS installed yet, in which case you end up with a dead NUC, since you can not see anything on the screen. Tried this will all the bios versions currently available - FY0015, FY0021 and FY0025. Tested with 2 different monitors. I am using the Kingston 4GB 1.35V DDR3L RAM, PN: KVR16LS11/4.
Solution i found is this:
1) Downgrade your BIOS to FY0015 as this work only in this version. To do this with a dead HDMI, you need to remove the bios security jumper and insert a flashdrive containing the FY0015.BIO file in the root. Wait a couple of minutes until the power button stops blinking, which indicates that the emergency bios recovery is complete. Turn off the NUC and reinsert the jumper. In case you already have FY0015 bios, you can skip this step.
2) After powering the NUC, keep pressing the F2 button, until the NumLock light on the keyboard goes off. This indicates that you entered the BIOS setup.
3) Press F9 to load the BIOS defaults and confirm with enter
4) Press F10 to save and exit and confirm with enter.
5) The NUC now restarts and the HDMI is back online.
I can change anything i like in the bios, except switching from UEFI to Legacy. I dont know if this bug is only in my machine or if it is a common problem but i sincerely hope it will be fixed ASAP, because i was almost going to RMA the damn machine till i found this solution.