I rather doubt anyone on staff will leap down the rabbit-hole of trying to describe something that complex to a novice user. It can be done, but it's one of those topics where if you need to ask how it's done you probably shouldn't be doing it.
Plan B: Move the data somewhere else. Reformat the drive. Move the data back .. this is much (much) easier.
Where do I start looking for a solution please?
What add-ons are installed?
I think their algorithm detects Kodi and assumes piracy. I've appealed it.
The appeal was successful so that cheap YouTube video on a borrowed TV has been reinstated 
I get this message during boot: "1.1784097 fail to initialize ptp_kvm". Maybe it's related to my problem?
100% sure this is harmless and not related.
It would need to be packaged into a Kodi binary add-on (as part of Hyperion would make sense) and then cross-compiled/built from source using the LE build-system. Then you install the binary add-on in Kodi. It's probably some way off from that point ..
Is it possible to install the OpenVPN server (not client!) under LibreELEC?
LibreELEC bundles the OpenVPN binary in the OS; it's entirely up to you whether you execute daemon instances with server or client (or both) configs, although 99% of the info you'll find in this forum solely discusses client matters.
Please update to a current LE12 nightly and see if the issue is resolved? (current nightlies are solid/stable to use)
The nature of support forums is that users who don't have problems generally don't show up and post about their lack of problems. Hence you don't find posts about LE11 and the MariaDB add-on. It works; there are probably a few thousand people just using it 
I pre ordered a Pi5 8GB so can't wait to see if it increases the menu/UI performance.
RPi4 wasn't really "slow" but there's an obvious increase with RPi5 
For me personally: I never turn the family daily-driver Pi off/on so the power button is irrelevant and while PCIe HATs will probably allow SSD boot in the near-term (which would be nice) I also like the passive-cooling flirc case which won't allow HATs to fit inside so I'll probably test it when someone ships a sample, and then continue to boot with an SD card. All the other hardware IO improvements are aimed at use-cases I don't have with LE; so apart from being able to handle some 4K content on YouTube the RPi5 is (for me) really just a faster RPi4 board. If you update from an older RPi2/RPi3 board you're still going to be happy with an RPi4 as it's a huge speed bump and improvement on them. Just don't let anyone show you an RPi5 for a while 
DV is encumbered by licensing and closed-source code needed to make things work. Kodi supports it on selected Android hardware like the nVidia shield and Firestick where the chip vendor paid-to-play and embedded the necessary stuff in the Android display pipeline; and support for DV is simply exposed to Kodi as a capability via the Android MediaCodec API.
None of the above happens on general purpose Linux or distros like LE because none of the moving parts are open-source and aside from a few scattered bits of boot and wireless firmware everyone middle-fingers closed source blobs and nobody will pay the licensing fees. The one possible exception you might look into is the latest OSMC Vero 5? box which is basically taking the Amlogic Android-Linux kernel (with all the needed codec support built-in) and perverting it for Linux/Kodi use. Go read the small print though; because I haven't and I'm only assuming it has support for DV media (but it likely does).
Have a read of this, it describes some of the main video setup things that help playback (even on TV's that are not 4K/HDR):
https://wiki.libreelec.tv/configuration/4k-hdr
To get 4K on the streaming services requires a certified device like AppleTV, Google Chromecast, Amazon firestick, or a more general Android device like nVidia Shield. Linux devices are almost non-existent so an LE/Kodi combination is better with offline media and DVB. Personally I'm okay with 720p/1080p for services via Kodi due to better privacy in the OS and apps, but I understand others are more about specs.
Most of the legal streaming services require Widevine L1 certificates to display higher resolution content, and RPi boards are not "certified" hardware and thus can only see the content available to L3 devices (we fake a web browser). This is generally 720p HD although Netflix can often show 1080p content when available too.
Are you using adjust-refresh and a mode whitelist to change-to/use the native resolution/refresh of the media being played?
2023-09-30 11:33:58.908 T:1502 ERROR <general>: EXCEPTION Thrown (PythonToCppException) : -->Python callback/script returned the following error<--
- NOTE: IGNORING THIS CAN LEAD TO MEMORY LEAKS!
Error Type: <class 'ModuleNotFoundError'>
Error Contents: No module named 'kodi_six'
Traceback (most recent call last):
File "/storage/.kodi/addons/plugin.audio.radio_data/addon.py", line 16, in <module>
from kodi_six import xbmc, xbmcgui, xbmcvfs, xbmcplugin, xbmcaddon
ModuleNotFoundError: No module named 'kodi_six'
-->End of Python script error report<--Looks like the add-on depends on a Python module (kodi_six) that isn't installed (or registered as a dependency). Best to report that to the add-on developer(s).
Great stuff.. enjoy
If the supply/demand ratio is as small for the Rpi5 as its been for the Rpi4, most of use won't get our hands on one for another year...
RPi5 will probably have an initial rush of orders and sell out, but within a month or so stocks should be back to normal again. RPi4 is generally available everywhere now, the supply issues have been resolved. See https://rpilocator.com/?cat=PI4
What should I do to get these dependencies?
Python2 support only exists in LE 9.2.x releases. LE10 and up use Python3 exclusively. You probably need to find a newer Python3 compatible version of the add-ons.
That warning is harmless and irrelevant because LE only uses pulseaudio for BT connections and everything else is native alsa. HDMI cardsnormaly have several card presentations to the OS including an analogue one; which will never support pass-through. Put Kodi in debug mode, restart, than "pastekodi" the entire logfile and share the URL; little snippets are rarely useful.
