Now it is solved: TrueHD and DTS-HD passthrough to my Denon under Win 10 and Librelec!
An update to HDMI Firmware for LSPCon 1.79.1.1 was the solution.
NUC7ixDN-HDMI_FW_Update_Tool-1.79.1.1
➢ Dolby Digital (AC3) capable Receiver ➢ ON
➢ Enable Dolby Digital (AC3) transcoding ➢ OFF
➢ Dolby Digital Plus - EAC3 ➢ ON
➢ DTS capable receiver ➢ ON
➢ TrueHD capable receiver ➢ ON
➢ DTS-HD capable receiver ➢ ON
Simple setup that worked with this passthrough (J3455_HDMI➢TV_HDM➢TV_Optical➢AVR_Optical):
➢ Dolby Digital (AC3) capable Receiver ➢ OFF
➢ Enable Dolby Digital (AC3) transcoding ➢ OFF
Simple setup that worked with this passthrough (J3455_HDMI➢TV_HDM➢TV_Optical➢AVR_Optical):
➢ Dolby Digital (AC3) capable Receiver ➢ ON
➢ Enable Dolby Digital (AC3) transcoding ➢ ON
Hi,
My Question:
Does anybody own a Asrock J3455-ITX with working TrueHD and DTS-DH passthrough?
➢If so, which LSPCON MCDP2800 firmware version is installed?
Does anybody own a NUC with a J3455
➢ with working TrueHD and DTS-DH passthrough?
➢ with NOT working TrueHD and DTS-DH passthrough but LSPCON MCDP2800 firmware >= 1.66 ?
I Own an Asrock J3455-ITX (Bios v1.80) since 5 years.
It was connected to a Philips 8503 and the S/PDIF of the TV with the AVR.
Simple setup that worked with this passthrough (J3455_HDMI➢TV_HDM➢TV_Optical➢AVR_Optical):
➢ Dolby Digital (AC3) capable Receiver ➢ OFF
➢ Enable Dolby Digital (AC3) transcoding ➢ OFF
Since LibreELEC 11 with HDR 
Now a new AVC (Denon x3800h) arrived.
J3455_HDMI➢AVR_HDMI➢TV_HDMI
An annoying HDMI handshake problem came up. A lot of reading an understanding followed.
Then most my test files worked with this settings:
Sync Playback to Display ➢ off
Delay after change of refresh rate ➢ 8 sec 
Adjust display refresh rate ➢ On start/stop
Whitelist ➢ (all 4K combinations)
But some files left unplayable, no difference with more then 8 sec delay after refresh rate change
➢ Playback start ➢ Black Screen ➢ Playback Continue ➢ Black Screen ➢ ...
So not a satisfying solution, HDMI handshake did not work.
So i found out that the MCDP2800 for DisplayPort1.2a-to-HDMI 2.0 is a big troublemaker (Bug 98797 - [BXT/KBL] - HDMI - HD audio passthrough dont work)
Happily a second SSD with Windows is available, so no big effort to update the firmware of the MCDP2800.
First try was to take one of the latest firmware versions (1.77) for the MCDP2800:
HDMI Firmware Update Tool for NUC6CAYH, NUC6CAYS
Started LibreELEC and all HDMI Handshake problems where gone.
Delay after change of refresh rate ➢ 0 sec
Even the unplayable files worked again.
Now the next step followed, but I got stuck!
The audio passthrough settings that are working/not working:
➢ Dolby Digital (AC3) capable Receiver ➢ ON
➢ Enable Dolby Digital (AC3) transcoding ➢ OFF
➢ Dolby Digital Plus - EAC3 ➢ ON
➢ DTS capable receiver ➢ ON
➢ TrueHD capable receiver ➢ OFF
➢ DTS-HD capable receiver ➢ OFF
➢ Core only ➢ ON
Next thought:
Using the v1.66 Firmware! Many NUC owners talked about v1.66 in context of a working HD-Audio passthrough.
But same result as before, nothing better or worse.
Next thought Linux vs. Win:
"Same" passthrough result with Win10 and VLC: No Atoms/TrueHD or DTS-HD
Reading the logs lead me also not into a helpful direction.
Nerveless here is a log with some try&error: LOG
Any help would be appreciated!
