I used an official DOS utility to update old signed BIOS on X10DRU-i+ to 3.5. Now everything is OK.
Disclaimer: At your own risk. If you use the wrong BIOS YOU WILL BRICK YOUR MACHINE.
I tried several times and finally it worked. ME switch should be in 2-3 during update: otherwise it looks working but it isn't. I manually rename fdt & afudos to exe, then I used default afudos switches from bat file:
afudos %1 /P /B /N /K /R /ME
and added /X /GAN.
Step-by-step:
1. PowerOFF server (remove from wall socket). ME jump to 2-3
2. in DOS boot (for official unsigned bios v.3.5 X10DRU2.427): download from official site, create FreeDOS usb stick by Rufus put unpacked files on stick.
3. Manually rename fdt & afudos to *.exe
3. Create a new bat file: fl35.bat with these 3 lines
fdt -w 50 A5
afudos X10DRU2.427 /P /B /N /K /R /ME /X /GAN
fdt -w 50 00
4. Boot from USB, run fl35.bat, wait!
5. PowerOFF. ME to 1-2. CMOS reset (by screwdriver)
6. PowerON, enter bios, F3 default, F4 save&exit
7. After reboot - setup bios properly.