I had a resolution problem too after using LibreELEC at Raspberry Pi (3B+) for some tests on a DVI monitor. noemin, which hardware do you use?
After a DVI session it had problems to use the (previously found and used) HDMI resolutions. The Raspberry Pi was booting in VGA mode on my HDMI TV. Some reboots, resolution changes and firmware updates (not sure what it was) solved the problem. Now the HDMI resolutions of the TV are detected right again, and it starts with the selected mode.
I think it's a bug on LibreELEC. It doesn't detect the proper resolutions when changing from DVI back to HDMI.