I would try adding hdmi_force_hotplug=1 to /flash/config.txt file. And if it does not help, see the post #6.
Thank you,
This worked out for me
at lease with the monitors that i have on hand
Quote from Da FlexYou have banned repos / add-ons installed. We will not give you support until you've been uninstalled them.
You are obviously right,
I didn't removed the repos /add-ons because it was obvious to me that it is a general issue and not related to any add-on