Install failure. It looks like LE can't see files from this repo as it is not showing size. I did update from old snapshot(the one from 10th). Will check with new installation as well.
Yes - it is upgrade related as fresh installation is working ok.
Okay, I investigated and all new images in the S8X2 project are affected: M8S+, MXIII*, Uni-n200.
The cause of the error is probably not in my own build, but rather in Kodi or LE.
It is a fact: The repository link for the devices has changed.
Old: http://libreelec.dtech.hu/packages/9.2.0/M8S-Plus/arm
New: http://libreelec.dtech.hu/packages/9.2.0/S8X2/arm
But that's still not the real reason, because both paths work and point to the same target (symlink).
2021-09-14 06:37:34.376 T:3010805760 WARNING: Repository add-on repository.libreelec.tv uses plain HTTP for add-on downloads in path http://libreelec.dtech.hu/packages/9.2.0/S8X2/arm - this is insecure and will make your Kodi installation vulnerable to attacks if enabled!
2021-09-14 06:38:04.143 T:2901406592 WARNING: Previous line repeats 32 times.
2021-09-14 06:38:04.143 T:2901406592 ERROR: Requested path http://libreelec.dtech.hu/packages/9.2.0/M8S-Plus/arm/virtual.dvb-tools/virtual.dvb-tools-9.2.0.105.zip not found in known repository directories
The repository link is good, the download link is working, it's just from the old index file.
Somehow the index would have to be updated to point to the new location, but I have no idea where LE stores this. 
Update #1:
It doesn't work either: https://wiki.libreelec.tv/how-to/force-add-on-update
It can be updated, but it will still use the old URL when downloading add-ons. What a stupid bug is this?
Update #2: The Kodi backup/restore solution does not work either.
