Solved XFCE4 doesn't start properly

That is interesting. Is the compositor in the autostart list? If the list can be reordered, perhaps moving it to the end may make a difference. I'm not sure if the autostart list is parsed/started from top to bottom, but if so it may help.
Going by hazy memory, I think ICE is related to components talking to each other. They register somehow and then can go talk to each other. If compositor is trying to talk through ICE to "A" but "A" hasn't registered yet, maybe compositor fails. That would match the workaround I think.

On the XFCE forum suggested a triplet of tuning to test if the compositor fails to start or not...

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So eventually the issue was the compositor, this command fixed it:
xfconf-query -c xfwm4 -p /general/vblank_mode -t string -s "off" --create

I bet this command makes the compositing CPU based... 🤔