I'm having the same issue as described here. I have a 10 year old Westinghouse TV, does 1080p. I had an older librelec that ran Kodi 18.9 on this TV and all was ok.
When I upgraded to 10.0.4, I see the rainbow square at boot, but then 'No Signal' If I plug my Pi into my son's monitor or my TV downstairs, I can see everything. I did set the resolution down to 1920x1080 while connected to something that works.
I've also added 'hdmi_force_hotplug=1' to my /flash/config.txt, but it didnt help. Besides, I thought I read that it was not supported anymore.
Any thoughts?
Regards