hi,
I think, I have got the same hdmi-output-problem - got an old monitor and since updating to libreelec 10, I get no signal on my monitor. with libreelec 9 evrything worked fine. I hope, I understood the discussion in this thread right, and so I tested the newest versions of libreelec 11 nightly build from the test.libreelec.tv website. I now testet libreelec 11 without and with adding hdmi_group=1 und hdmi_mode=20 at the end of config.txt. the logs are at http://ix.io/4jY1 without and http://ix.io/4jY5 with the config.txt changes. I still got no picture on my monitor!
maybe this post is helpful for solving these problems! if not, I may remain on libeelec 9.
best regards Michael Suhl