The TV resolutions and years of production are important to answer this.
When people start using the RPi 4B on a modern 4K TV, then often the HDMI cable is too old. Be sure you are using an HDMI 2.0 or 2.1 standard cable to be ready for 4K.
If your non-working TV is very old (first flat screens), then it could be necessary to enforce a video mode at config.txt. Read here.
I think I'll have a play with overscan and forcing the frame buffer size and see what comes out.
Exactly. Disable overscan and make frame buffer size fit to your selected resolution.
Try LE's screen calibration whenever you check a new video mode.
Here is another config.txt setting for an older TV, which you can try: click. Maybe it does the trick for the final half inch.
Another short research says LE is using samba.conf instead of smb.conf.
Create /storage/.config/samba.conf and increase aio read size and aio write size.
Read the smb.conf documentation and find the equivalent variables at samba.conf.
Sounds like a buffering issue. I don't use Samba, but a short search says that Samba has socket options with SO_RCVBUF and SO_SNDBUF buffer size values. Find smb.conf on LE and increase those values.
Just yesterday I've been installed libdvdcss on my Ubuntu. It needs two commands at root mode:
apt install libdvd-pkg
dpkg-reconfigure libdvd-pkgHaving those libraries on your build system hopefully makes the "dvdcss-autoreconf" call successful.
It's "HD ready".
Some TV's have picture optimization modes, which increases the picture size to outside the screen (not much, but visible).
Check your TV settings, and disable all picture optimization modes you have.
Have you tried a different HDMI cable? 4K needs an HDMI 2.0 or 2.1 standard cable, because of higher data rate.
Perhaps you can create a bootable USB stick with Ubuntu 18.04.4 or upwards.
I agree. That's a good reference point to start with.
Additionally try to access your BIOS / EFI menu. Maybe you'll find some tweaks there.
Usually missing sound will not stop video playback on LE.
Hardware specs are interesting for me. I remember a user with a really tiny PC, and CPU performance mode was BIOS-blocked to avoid heat issues. Tiny PC's often want to switch to performance mode after some seconds of video playback, like described here.
On most Linux flavors libdvdcss isn't part of the system due to a missing license. You maybe have to download it manually, and put it into the right folder.
I did a minimal image ubuntu install and now running kodi as a service with no WM. Solid so far and with the added bonus of proper sound again.
That's a proof of my theory. I think it's a timing issue. The more processing the PC has to deal with, the longer the USB latency. At some cutting point, some audio data will be dropped, because there is no chance to deliver in time.
I'm currently running a real-time-kernel Ubuntu for music production with a connected USB-MIDI controller. The USB latency difference between a real-time / non-real-time system is significant. I know what I'm talking about.
-> Stay away from high speed USB audio if you run a regular (non-real-time-kernel) Linux.
My theory: You had a power outage, which turned on your TV by a CEC signal that came from the RPi. The power outage was leading to a bad HDMI handshake, resulting in a wrong HDMI mode.
Things you could do to avoid such situation:
PS: It's like in that "Poltergeist" movie. 
I've never seen such an error in this forum. Is it a reproducible error? If yes, describe all steps you did.
That's great, Denis! Another laptop stays alive. 
You can mark the thread as solved now.
RPi and PC are both USB host devices. You want to use your RPi in USB client mode, but that's impossible.
Blacklist the nouveau driver like this:
APPEND boot=UUID=<YOUR_BOOT_UUID> disk=UUID=<YOUR_DISK_UUID> quiet initcall_blacklist=nouveauIf this doesn't work, try nouveau_init instead of nouveau at the append line.
It's not tested by me, I'm just an RPi user.