HDMI is a standard, and the only parameter that could really change between devices is data transmission speed.
My advice is to take the HDMI cable from the working TV, and plug it into the not working one. Just to be sure it's not the cable. As you know, some HDMI cables are ready for high speed, and others are not, so CEC data could get lost.
I had the safe mode bug today at LE 9.1.002 (RPi3B+).
It happened after I've been added the Kodi Nerds repo, and installed / uninstalled "Amazon Prime Music" from there.
After shutdown and power on I went into safe mode.
I did a reboot from SSH, and normal mode worked again. Maybe it helps to find the bug.
...such a shame i have to return this pi now and wait for a proper build to come out first.
Not so fast! RPi4B is a brand new device, which takes time at the software part. Don't forget it's made by hobby programmers.
Thanks for the info. The RPi4B is a brand new device. Just wait until all the little things will be fixed.
Also you can do a formal bug report, or just mention it at News and Announcements, where some developers are reading, too.
I do something similar, but just to reboot Kodi. Since I use another programs in background and If I not reboot Kodi will be slowly with the past of days. What I wished is do this, automatically, based on tv power on/off, and just Kodi, not LibreELEC.
But I think this is impossible.
I think it could work. Use the RPi always-on for CRON, and switch Kodi / tvservice (HDMI) on / off with a button connected to GPIO.
I haven't seen any related errors on your log. Maybe the MKV's have different horizontal resolutions, and the filled space is just black on your other players. Look at the MKV's meta data for differences.
I found this, but how I can do this automatically for when I turn tv on?
The major problem is that you disable CEC when you switch HDMI off.
If you're familiar with electronics, you could install an infrared receiver element on your RPi. So you can turn HDMI on by IR code.
An easier solution is to install a push button, and listen to it by an autostart script (similar to the well known power button on RPi).
Take into account that an RPi Zero is cheap, and you can even run LE on it just for CRON jobs.
To run LE headless, switch HDMI off from autostart.sh. As I already mentioned, switching off works.
In theory you can switch HDMI off and on by using the "tvservice" command with the appropriate options. That would save energy. I was playing around with that command a couple of month ago, but I was not successful to switch HDMI on after switching it off. Maybe it's working with the current LE version.
Or you could buy another RPi, and run one of them headless (Raspbian) for your CRON services. Install a power button on the other one, where you run LE.
An RPi2 is definitely the wrong hardware to display such high resolutions. So, it's not a bug, it's a feature to keep it disabled. 
I had a safe mode issue, too, shortly with LE 9.1.002. I think it was triggered by a settings change I made.
I have been changed "Settings -> Services -> General -> Device name". My advice is to keep this setting untouched at "Kodi".
I use analogue output a lot connected to an old Dvd Surround Sound System and audio is normal as when playing dvd's..
I always leave the audio setting ON to both analogue and hdmi audio...then when I need to use the speakers just mute the TV.
My RPi analogue audio has normal volume, too. It seems like only robmcc83 has such a problem.
I haven't found that option inside guisettings.xml. If you really want to force 5GHz usage, you have to compile your own version of LE.
You can try to find a channel number inside the 2.4GHz frequency band, which is not overloaded by other WLAN users (router option).
It just shows bad forum sw or at least spam protection. I don't see as much spam on other forums I visit. There is zero spam on a most of them actually.
Yes, I've never seen that much spam in other forums, too. Maybe LE is a threat for Chinese settop box companies.
I am already in creating blocklists for that Chinese symbols, sadly thats pretty difficult
Good man. I've been suggested that a while ago, too. That Chinese spam is a pain in the ass.
My last resort is to try a fresh install of LibreELEC just incase any settings have been changed that have caused this.
That is the last thing I can try.
Yes, that's my final thought, too. During the updates an analogue setting maybe got lost. Analogue output is not a very popular feature.
Here's a USB Audio Adapter that does not hog room from your usb jacks
Yes, that's cheaper, but he was talking about "aesthetics of the case". So let's see if he invests the extra money for that reason. Just an HDMI cable looks better for sure.
Hopefully the case has no additional pass-through resistor at the audio jack.
Tried your guisettings.xml and it's exactly the same, still low audio via analogue unfortunately.
OK. Next theory: The analogue audio hardware has been damaged, and your other OS compensates it by doing extra amplification.
To give you a more practical advice: You could buy an HDMI audio extractor.
Benefits:
