Is the hardware on RPI 3 B+ enough for playing heavy videos?
Some stream providers (Netflix) use software decoding, which will work at lower resolutions only (720p / HD-Ready). Other providers (Amazon Video) use hardware decoding, which needs less performance and runs at 1080p / Full-HD.
If you need EVERYTHING at Full-HD or better, buy the Nvidia Shield TV.
is there any problem if I just clone the sd card from one raspberry to the other ?
No, if you don't use a static IP in your LibreELEC configuration. Non-static DHCP will let your router select a unique IP. That's all you need in your local network.
This is a classic client/server scenario. Try to find an add-on, which delivers remote access to your files (maybe NFS). So, one Raspberry Pi will be a client/server, and the other one just a client.
It turned out that I had a bad HDMI cable. Since using a new HDMI cable, the bug doesn't appears anymore.
My theory: The bad cable has some lose inner contacts, because the outer contacts are all gold-plated. The lose contacts created an incomplete whitelist on guisettings.xml. The broken HDMI connection was unable to transfer the proper whitelist to Kodi. Problem solved.
To search for the error systematically, the first thing is to find a way to see all incoming CEC data. The LibreELEC CEC add-on should give some output.
After 20+ reboots / shutdowns the error does not appears in LibreELEC 8.90.006 anymore. Thanx for fixing the bug!
Yes, I know about the limited region change abilities of DVD drives. The external DVD drive that I had for testing (I gave it back later), only had to initialize a region code at first usage: It didn't worked on Ubuntu /VLC, but after initializing it with "Windows DVD-Player", it worked on Ubuntu / VLC, too, and never gave a prompt when switching between RC1 / RC2 DVD's.
The reason why I gave the external DVD drive back (and never tried it on Raspberry Pi) was, that some DVD's of the same region worked, but others did not. So, I came to the testing result, that it's not the region code, but the CSS encryption, which is the real troublemaker. Different DVD drives show different results for the same DVD.
I would try the stable LibreELEC-9.0 update channel. Post the log file, if the stable release doesn't work. See here how to post kodi.log.
Good. Your are the thread starter, so it's your turn to mark your thread as solved.
"Spins up" and "has enough power to be used" are two entirely different things.
True. I currently have an HDD, which spins up, but will not be detected anymore by my Ubuntu Linux. Spinning and using are two different levels of energy requirement.
It probably doesn't get enough power over USB. Every detected device will be visible on the /dev folder (like sda, sdb... use SSH login to see it). If it doesn't appears in that folder, then it's definitely an USB power issue.
The bug doesn't appear on LibreELEC 8.90.006 so far. 
I think it's solved, but I will do some more shutdowns/reboots before I mark this thread as solved.
Thanx for the info. I will close my bug report thread, when I reach the 20+ reboot/shutdown count without errors.
I'm not here to read your handbook. I can't help you anymore, because you don't answer my questions, and you don't follow my suggestions.
....hmm i guess that is not really the problem.
Could you explain what's the problem with your beamer handbook? I don't get it. I think it's important, what your beamer supports.
Can i check somehow if the graphics or the hdmi port is still working correctly?
No TV or DVI monitor available to test? It's pretty basic to test the hardware first.
Today I was able to test a USB DVD drive on my Ubuntu Linux PC (VLC player), using region code 1 and 2 DVD's. Some DVD's work, some don't. On Windows 10 (64 bit), using the built in "Windows DVD-Player", some DVD's don't work, too.
Funny thing: My internal PC DVD player works with some DVD's, who are unable to start in the external DVD player. I think the DVD copyright protection produces different results on different DVD drives.
So, there is no more need to try the USB DVD drive on Raspberry Pi - the drive failed. It's a game of luck (different DVD copyright protection), if some DVD works or not.
I have to use a stand-alone region-free DVD player. It's a libdvdcss and hardware issue, not a LibreELEC issue. Problem solved.
Codestate 0x40001 [NTSC 4:3], 720x480 @ 60.00Hz, interlaced...The configs don't seem to work ....
Are you sure your beamer can deal with interlaced signals? If you still have the handbook, investigate what display modes are supported by the beamer. Here is my guisettings.xml, which uses 720p / 60Hz as default. You can try those settings on your device.
Check out the whitelist on your guisettings.xml:
<setting id="videoscreen.whitelist">.....</setting>Maybe the detection of available screen modes doesn't works right with your beamer. Detected screen modes will appear on that whitelist. Manually editing the whitelist (or using my guisettings.xml) could be worth a test, too.